Thermoelectric devices having reduced thermal stress and contact resistance, and methods of forming and using the same

ABSTRACT

A method includes preparing a thermoelectric material including p-type or n-type material and first and second caps including transition metal(s). A powder precursor of the first cap can be loaded into a sintering die, punches assembled thereto, and a pre-load applied to form a first pre-pressed structure including a first flat surface. A punch can be removed, a powder precursor of the p-type or n-type material loaded onto that surface, the punch assembled to the die, and a second pre-load applied to form a second pre-pressed structure including a second substantially flat surface. The punch can be removed, a powder precursor of the second cap loaded onto that surface, the first punch assembled to the die, and a third pre-load applied to form a third pre-pressed structure. The third pre-pressed structure can be sintered to form the thermoelectric material; the first or second cap can be coupled to an electrical connector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. §120 of U.S. patentapplication Ser. No. 14/469,404, filed on Aug. 26, 2014 and entitled“Thermoelectric Devices Having Reduced Thermal Stress and ContactResistance, and Methods of Forming and Using the Same,” the entirecontents of which are incorporated by reference herein, which claimspriority to the following applications, the entire contents of both ofwhich are incorporated by reference herein:

U.S. Provisional Patent Application No. 61/872,745, filed Sep. 1, 2013and entitled “Thermal Stress and CTE Mismatch Management inThermoelectric Package;” and

U.S. Provisional Patent Application No. 61/955,323, filed Mar. 19, 2014and entitled “Bulk Metal-Capped Thermoelectric-Composite SandwichStructure and Method Thereof.”

BACKGROUND

The present invention is directed to structure and method ofthermoelectric device package according to certain embodiments. Moreparticularly, some embodiments of the invention provide structure andmethod for reducing thermal stresses in thermoelectric legs duringmanufacturing a thermoelectric device. Merely by way of example, it hasbeen applied for using CTE (coefficient of thermal expansion) matchingductile materials at the interface between thermoelectric materials andshunts on ceramic base plate. It would be recognized that the inventionhas a much broader range of applicability.

Additionally, the present invention is directed to thermoelectriccomposite structures and methods of making the same according to someembodiments. More particularly, certain embodiments of the inventionprovide a bulk thermoelectric composite material capped with two metallayers forming a structure for the manufacture and assembly ofthermoelectric elements for large-scale thermoelectric power systems.Merely by way of example, the invention presents a method ofco-sintering powdered thermoelectric composite materials with powderedmetal material on top and bottom to form a bulk metal-cappedthermoelectric sandwich structure capable of making a plurality ofthermoelectric elements that may be sorted by thermoelectric performancefor the manufacture of a custom-scaled thermoelectric module and aremechanically and electrically robust. However, it would be recognizedthat the invention has a much broader range of applicability.

Thermoelectric devices are often packaged using a plurality ofthermoelectric legs arranged in multiple serial chain configurations ona base structure. Each of the plurality of thermoelectric legs is madeby either p-type or n-type thermoelectric material. The thermoelectric(TE) material, either p-type or n-type, is selected from semiconductorcharacterized by high electrical conductivity and high thermalresistivity. One p-type leg is pairwisely coupled to one n-type leg viaa conductor from each direction in the serial chain configuration, oneconductor being coupled at one end region of the leg and anotherconductor being coupled at another end region of the leg. When thethermoelectric device is applied with a bias voltage across thetop/bottom regions using the two conductors as two electrodes, atemperature difference is generated so that the thermoelectric devicecan be used as a refrigeration (e.g., Peltier) device. When thethermoelectric device is disposed to a thermal junction with conductorsat first end regions of the legs being attached to a cold side of thejunction and conductors at second end regions of the legs being incontact with a hot side of the junction, the thermoelectric device isable to generate electrical voltage across the junction as an energyconversion (e.g., Seebeck) device.

The energy conversion efficiency of thermoelectric devices can bemeasured by a so-called thermal power density or “thermoelectric figureof merit” ZT, where ZT is equal to TS²σ/k where T is the temperature, Sis the Seebeck coefficient, σ is the electrical conductivity, and k isthe thermal conductivity of the thermoelectric material. In order todrive up value of ZT of thermoelectric devices utilizing the Seebeckeffect, on the one hand, efforts are made for searching high performancethermoelectric materials and developing low cost manufacture processes,on other hand, efforts also are needed for improving thermoelectric legpackaging techniques whenever the any high-performance thermoelectricmaterials are available. Additionally, there are needs for improvingmanufacturability of the high performance bulk-size thermoelectric legsthat are packaged into thermoelectric devices.

For example, different types of materials may be provided for forming por n type legs and for forming cold or hot-side contacts. During apackage process to assemble them together, large thermal stresses duringdevice operation would be a key problem to overcome, e.g., when morethermoelectric applications push the hot-side temperature exceeding 600°C. Further, bond strength between different thermoelectric materials inpackage would also be one of the big issues, as newly developedthermoelectric material combinations and new operation environmentrequirements raise more challenges as well as opportunities.

For example, the thermoelectric legs made of or including either n-typeor p-type thermoelectric materials integrated with conductive materialas an electrical contact at both ends before the legs (on a size scaleof a few millimeters cubed) can be diced from a larger raw pellet.Conventionally, a series of sputtered or evaporated conductive thinfilms are formed on a thermoelectric material to make electrical contactto the thermoelectric elements for assembling into a module. There aremultitudes of challenges with such thin films on various thermoelectricmaterials with high performance properties. Most issues include filmdelaminating and cracking or the whole piece of thermoelectric materialcracking during the formation of individual thermoelectric legs orelements. The thin film electrical contacts mentioned above are used forcontact and bonding but they are too thin by themselves to allow directmeasurement of the electrical properties of the thermoelectric material.The thin film does not provide low enough electrical resistance to allowfor sufficient current spreading across the leg cross-section. Thisresults in a high resistance measurement and one that could differdepending on where on the surface the measuring probe makes contact.

It is commonly known that the properties of thermoelectric materialsvary throughout a large sample such that small subsamples diced from theas-manufactured large sample have a wide range of properties compared tothose measured from the large sample. Many thermoelectric materials aremechanically brittle, prone to cracking as thickness of the manufacturedpellet changes, and hard to dice into the final thermoelectric legs withdesired form factor without breaking Even though a relatively largepiece of thermoelectric material with metal contacts can be made, itselectrical properties can only be estimated on average as a whole pieceof material without being able to properly determine the individual legperformance after dicing. For a thermoelectric material with thin filmelectrical contacts, binning of the thermoelectric legs by selectedperformance level is not possible prior to building the fullthermoelectric system.

Merely as an intermediate material, the thin film electrical contactsmentioned above also suffer from poor electrical conduction due to filmcracking, parasitic resistances, and bond failure, leading to poorelectrical integration when assembling into thermoelectric powergeneration systems. Alternatively, co-sintering of metal foils withthermoelectric materials has been attempted, but the metal foils oftenexperience problems of poor bonding with the thermoelectric material.

SUMMARY OF THE INVENTION

Therefore, it is desired to have improved techniques for packagingthermoelectric legs with reduced thermal stress associated with newlydeveloped composite thermoelectric leg materials and simplifying processsteps for making high performance thermoelectric device with low cost.Details about one or more techniques using coefficient of thermalexpansion (CTE) matching across bonding regions of each thermoelectricleg for packaging thermoelectric devices are presented as variousembodiments throughout this specification and particularly below.

Additionally, it is highly desirable to develop an improvedthermoelectric composite material to overcome thermoelectric materialcracking (during either sintering or dicing), poor adhesion betweenmetal contacts with the thermoelectric material, variation inperformance across relatively large pellet, and to provide a method formaking the same thermoelectric composite material with the ability tomeasure the properties of the small diced thermoelectric elements priorto installing them in the power generation system. Many benefits areexpected upon the application of various embodiments of the presentinvention. One of them is the ability to control thermoelectric propertyvariations out of the sintered thermoelectric composite material pelletdue to defects, agglomerates, presence of various material phases,variations of component concentrations, and other stochastic phenomena.Another benefit of certain embodiments of the present invention lies inthe enhancement of mechanical robustness of individual thermoelectriclegs diced from the co-sintered pellet with the metal-cappedthermoelectric composite structure. An alternate benefit of someembodiments of the present invention also lies in the enhancement ofelectrical robustness of individual thermoelectric legs diced from theco-sintered pellet with the metal-capped thermoelectric compositestructure. Details about the improved structure and related manufacturemethod are presented below.

Under one aspect, according to one embodiment, a method of forming athermoelectric device includes preparing a thermoelectric materialincluding a p-type or n-type material and first and second capsrespectively including first and second cap materials respectivelydisposed on either side of the p-type or n-type material, the first andsecond cap materials each respectively including an independentlyselected transition metal. Forming the thermoelectric material caninclude loading a powder precursor of the first cap material into asintering die; assembling one or more punches to the powder precursor ofthe first cap material in the sintering die; and applying a firstpre-load via the one or more punches to the powder precursor of thefirst cap material to form a first pre-pressed structure including afirst substantially flat surface. Forming the thermoelectric materialfurther can include removing a first punch of the one or more punches toexpose the first substantially flat surface; loading a powder precursorof the p-type or n-type material into the sintering die and onto theexposed first substantially flat surface; assembling the first punch tothe powder precursor of the p-type or n-type material in the sinteringdie; and applying a second pre-load via the one or more punches to thefirst pre-pressed structure and the powder precursor of the p-type orn-type material to form a second pre-pressed structure including asecond substantially flat surface. Forming the thermoelectric materialfurther can include removing the first punch to expose the secondsubstantially flat surface; loading a powder precursor of the second capmaterial into the sintering die and onto the exposed secondsubstantially flat surface; assembling the first punch to the powderprecursor of the second cap material in the sintering die; and applyinga third pre-load via the one or more punches to the second pre-pressedstructure and the powder precursor of the second cap to form a thirdpre-pressed structure. Forming the thermoelectric material further caninclude sintering the third pre-pressed structure to form thethermoelectric material; and coupling at least one of the first andsecond caps of the thermoelectric material to an electrical connector.

Under another aspect, according to another embodiment, a thermoelectricdevice includes a thermoelectric material including a p-type or n-typematerial and first and second caps respectively including first andsecond cap materials respectively disposed on either side of the p-typeor n-type material, the first and second cap materials each respectivelyincluding an independently selected transition metal. The thermoelectricmaterial can be formed by co-sintering a powder precursor of the firstcap material, a powder precursor of the p-type or n-type material, and apowder precursor of the second cap material in a sintering die. Aparticle size ratio of the powder precursor of the p-type or n-typematerial to the powder precursors of the first and second cap materialscan be in the range of approximately 1:1 to approximately 1:50. Thedevice also can include an electrical connector, at least one of thefirst and second caps of the thermoelectric material being coupled tothe electrical connector.

Under yet another aspect, according to yet another embodiment, a methodof forming a thermoelectric device can include providing athermoelectric material including a p-type or n-type material and firstand second caps respectively including first and second cap materialsrespectively disposed on either side of the p-type or n-type material,the first and second cap materials each respectively including anindependently selected transition metal, wherein a thickness of each ofthe first and second caps is approximately 0.2 mm to approximately 2 mm.The method further can include dicing the thermoelectric material toform a plurality of individual thermoelectric legs; testing anelectrical resistance of each of the individual thermoelectric legs;sorting the individual thermoelectric legs based on the testedelectrical resistance; and coupling at least one of the first and secondcaps of at least one of the sorted individual thermoelectric legs to anelectrical connector.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a thermoelectric device package.

FIG. 2 illustrates results of an exemplary 3-dimensional (3D) stressmapping calculation performed on the thermoelectric device package ofFIG. 1.

FIG. 3 illustrates a thermoelectric device package according to certainembodiments of the present invention.

FIGS. 4A-4D illustrates results of an exemplary 3D stress mappingcalculation performed on illustrative configurations of thethermoelectric device package of FIG. 3, according to certainembodiments of the present invention.

FIGS. 5A-5B are photographic images of thermoelectric legs according tocertain embodiments of the present invention.

FIG. 6 illustrates steps in an exemplary method for forming athermoelectric device according to certain embodiments of the presentinvention.

FIG. 7 is a plot illustrating the number of useful thermoelectric legsformed by dicing a thermoelectric material according to certainembodiments of the present invention.

FIG. 8 illustrates steps in an exemplary method for forming athermoelectric material according to certain embodiments of the presentinvention.

FIGS. 9A-9B are scanning electron microscope (SEM) images of anexemplary thermoelectric material according to certain embodiments ofthe present invention.

DETAILED DESCRIPTION

The present invention is directed to structure and method ofthermoelectric device package according to certain embodiments. Moreparticularly, some embodiments of the invention provides structure andmethod for reducing thermal stresses in thermoelectric legs duringmanufacturing of a thermoelectric device. Merely by way of example, ithas been applied for using CTE (coefficient of thermal expansion)matching ductile materials at the interface between thermoelectricmaterials and shunts on ceramic base plate. It would be recognized thatthe invention has a much broader range of applicability.

Additionally, the present invention is directed to thermoelectriccomposite structures and methods of making the same according to someembodiments. More particularly, certain embodiments of the inventionprovide a bulk thermoelectric composite material capped with two metallayers forming a structure for the manufacture and assembly ofthermoelectric elements for large-scale thermoelectric power systems.Merely by way of example, the invention presents a method ofco-sintering powdered thermoelectric composite materials with powderedmetal material on top and bottom to form a bulk metal-cappedthermoelectric sandwich structure capable of making a plurality ofthermoelectric elements that may be sorted by thermoelectric performancefor the manufacture of a custom-scaled thermoelectric module and aremechanically and electrically robust. However, it would be recognizedthat the invention has a much broader range of applicability.

For example, embodiments of the present invention provide thermoelectricdevices having reduced thermal stress and contact resistance, andmethods of forming and using the same. In an illustrative embodiment, athermoelectric device can include a thermoelectric material thatincludes p-type or n-type material and first and second capsrespectively including first and second cap materials respectivelydisposed on either side of the p-type or n-type material. The first andsecond cap materials each can respectively include an independentlyselected transition metal. As described in greater detail herein, themethod by which the thermoelectric material is made, or as the materialsused to form the p-type or n-type material, or the dimensions of thematerials, or any suitable combination of the foregoing can reducemismatch in coefficient of thermal expansion (CTE) between the p-type orn-type material and the first and second cap materials, can improvebonding between the p-type or n-type material and the first and secondcap materials, can improve durability of the thermoelectric device, canimprove resistance of thermoelectric legs formed using thethermoelectric material, and can improve the percentage ofthermoelectric legs formed using the material that have a satisfactoryresistance.

For example, FIG. 1 is a schematic diagram of a structure 1000 includinga pair of thermoelectric (TE) unicouples disposed on a base structureand capped by a shunt material according to a thermoelectric package,and FIG. 2 illustrates results of an exemplary 3-dimensional (3D) stressmapping calculation performed on the thermoelectric package of FIG. 1.Presenting the TE structure 1000 and its subsequent thermal stressanalysis is utilized as part of the invention process for developing theimproved CTE matching thermoelectric leg package and associated methodsaccording to certain embodiments as described elsewhere herein.

As shown in FIG. 1, the two redundant pair of TE unicouples, or a TEunit 1000 including four TE legs, are disposed with alignment ontorespective locations of a base plate 110. In the illustrated embodiment,the base plate 110 is an electrical insulator material but withrelatively good thermal conduction characteristics. In an example, thebase plate includes, or is made by, a ceramic material, such as siliconnitride Si₃N₄ material. In another specific example, as shown in FIG. 1,a metal pad 111 is inserted between a first end region 101 of each TEleg 100 (e.g., a p-type or n-type material) and the base plate 110. Onthe same location but the opposite side of the base plate 110, anothersimilar metal pad 112 is also attached as a hot-side thermal contact forconducting heat from a heat source for the thermoelectric unit 1000.Physically it also provides a way that can at least partially balancethe stress within thickness of the base plate 110. Additionally, thesecond end region 102 of each TE leg 100 is bonded with a conducting,e.g., electrically conducting, shunt plate 120 near its four cornerregions. Specifically, in one embodiment, the shunt plate 120 isdesignated for attaching a cold-side heat exchanger (from above, but notshown). For improving its flexibility and so as to partially relievesome thermal stress, in an illustrative embodiment the shunt plate 120is made by a piece of metal foil or thin plate having an opened centralwindow 121. Copper or other relatively good electrical and thermalconductor would be preferred choice of material for the shunt plate 120.

However, due to CTE difference between the TE leg material and the metalpad material as well as the base plate material, thermally inducedmismatch stress within the TE leg can be relatively, or very, large.FIG. 2 shows an exemplary result of a 3D stress-level mapping throughoutthe two-pair TE unicouples (or a 4-leg unit) of FIG. 1, calculated usingfinite element analysis (FEA) using SolidWorks Simulation (DassaultSystemes SolidWorks Corporation, Waltham, Mass.). As shown, thethermally induced stress is quite large across the whole unit asindicated by relatively large (in the range of 240 MPa) and relativelyunevenly distributed, e.g., near the junction region near the metal pad111 and the first end region 101 of each TE leg 100. The result isobtained by assuming the base plate 110 with its bottom metal pad 112attached to a hot-side heat source during a standard TE application. Asthe heat source temperature has been raised higher and higher in manydesired applications, the thermal stress level could be even higher. Formany desired TE applications, the hot-side temperature is above 600° C.which may cause substantial performance degradation or potential devicefailure. As shown in FIG. 2, the metal pad 111 below the first endregion 101 of each TE leg 100 has larger expansion than the latter,causing uneven stress distribution especially at regions near four lowercorner edges.

As noted further above, some embodiments of the present inventionprovide thermoelectric devices having reduced thermal stress and contactresistance, and methods of forming and using the same. In anillustrative embodiment, a thermoelectric device can include athermoelectric material that includes p-type or n-type material andfirst and second caps respectively including first and second capmaterials respectively disposed on either side of the p-type or n-typematerial. The first and second cap materials each can respectivelyinclude an independently selected transition metal. For example, FIG. 3is a schematic diagram of a pair of thermoelectric (TE) unicouplesdisposed on a base structure with an intermediate ductile cap insertedfor each TE leg according to an embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. The figure is merelyan example for one of several possible configurations.

In the illustrated embodiment as shown in FIG. 3, certain aspects of theTE unit 2000 are substantially similar to the TE unit 1000 except that aductile cap 140 is inserted between the first end region of the TE leg100 (e.g., a p-type or n-type material) and the metal pad 111 on thebase plate 110. Another ductile cap 140 can be inserted between thesecond end region of the TE leg 100 and the metal pad 102 coupled to theshunt. In particular embodiments, the ductile cap(s) 140 independentlyare configured to be or to include a material or composite material thathas a coefficient of thermal expansion (CTE) substantially matching withthe TE material above, which can be a p-type or n-type material, so thatthe thermal induced CTE mismatch stress within the TE leg 100 issubstantially reduced. The material(s) of the ductile cap(s) can beselected so as to respectively include a CTE that differs by 20% or lessfrom a CTE of the TE material (e.g., p-type or n-type material), or by10% or less from a CTE of the TE material, or by 5% or less from a CTEof the TE material, or by 2% or less from a CTE of the TE material, orby 1% or less from a CTE of the TE material. For example, thematerial(s) of the ductile cap(s) can be selected so as to respectivelyinclude a CTE that differs by 5 ppm/° C. or less from a CTE of the TEmaterial (e.g., p-type or n-type material), or by 4 ppm/° C. or lessfrom a CTE of the TE material, or by 3 ppm/° C. or less from a CTE ofthe TE material, or by 2 ppm/° C. or less from a CTE of the TE material,or by 1 ppm/° C. or less from a CTE of the TE material.

Exemplary TE materials (e.g., p-type or n-type material) suitable foruse within TE unit 2000 include, but are not limited to, silicide-based(e.g., Mn_(x)Si, Mg₂Si, or Mg₂SiSn), skutterudites, bismuth tellurides,lead tellurides, tin tellurides, silicon germanium, zinc antimonide,tetrahedrite, TAGS (Te—Ag—Ge—Sb), Zintl, tin selenides, lanthanumtellurides, and the like. In one illustrative example, the p-type orn-type material can include magnesium silicide or manganese silicide. Inanother illustrative example, the p-type or n-type material can includetetrahedrite or Mg₂SiSn.

Depending on choice of the TE material for different TE application, aTE material with lower CTE (6-8 ppm/° C.) can be matched with cap(s),e.g., ductile cap(s), having similar CTE that is made from one or morematerials selected from Kovar, Cr, Molybdenum, Ni—Fe alloy, Cu—Mo alloy,and the like. In certain embodiments, the one or more cap(s) materialsare selected from Kovar, Cr, Molybdenum, 50/50 Ni—Fe alloy, and thelike. In another example, a TE material with higher CTE (13-17 ppm/° C.)can be bonded with caps, e.g., ductile cap(s), having matched CTE madefrom one or more materials selected from Ni, Monel, Dura Nickel, Cu—Nialloy, Cu—Mo alloy, Fe, and the like. In certain embodiments, the one ormore caps(s) materials are selected from Ni, Monel, Dura Nickel, Cu—Ni30, Cu—Ni 10, and the like. In one nonlimiting example, the p-type orn-type material includes magnesium silicide, and the cap(s) includenickel. In another nonlimiting example, the p-type or n-type materialincludes manganese silicide, and the cap(s) include chromium. Othercombinations of materials suitable can be selected. The CTEs of variousTE materials and materials suitable for use in cap(s) are known in theart or can be readily determined. Note that the caps(s) need notnecessarily be formed of the same material as one another, and indeedcan include independently selected materials that are different than oneanother. Additionally, in an illustrative embodiment, the materials ofthe cap(s) do not include a silicide.

In an exemplary embodiment, the ductile cap(s) 140 each can be or caninclude a separate piece of composite material by sintering severalabove selected materials, which also provides an enhanced bondingstrength between the TE leg and the metal pad below as those materialswithin the ductile cap(s) provide elements for facilitating metalbrazing process. Particularly, the bonding strength at the hot-sidejunction is retained during those designated high-temperature operationenvironment. Alternatively, the ductile cap(s) 140 can be added directlyto the first end region of the TE leg as part of the formation processto fabricate each TE leg 100 by sintering process. In other words, theductile cap(s) 140 can become part of the TE leg 100.

During any TE operation, the intermediate cap(s), e.g., ductile metalcaps, can or will deform to accommodate CTE mismatch between the ceramicbase plate and TE leg. Especially near the hot-side junction, thethermal expansion can be the greatest for each TE leg. Similarly, nearthe cold-side junction, the thermally induced mismatch stress betweenthe TE leg and the shunt plate also can exist. As mentioned furtherabove, another CTE matching metal cap can be inserted between the secondend region of the TE leg and the shunt material or co-sintered with TEmaterial on the second end region of each TE leg during the TE legformation process. As a result, a thermoelectric package with p-type andn-type materials with same and different CTEs can be mounted on the samebase plate or shunt substantially without inducing thermal stresses fromCTE mismatch in the legs.

FIGS. 4A-4D illustrate results of an exemplary 3D stress mappingcalculation performed on illustrative configurations of thethermoelectric device package of FIG. 3, according to certainembodiments of the present invention. These diagrams are merelyexamples, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, FIG. 4D shows an exemplary result of 3Dstress-level mapping throughout the two-pair TE unicouples (or a O-legunit) after introducing a ductile cap 140 inserted between the first endregion of the TE leg 100 (e.g., a p-type or n-type material) and themetal pad 111 on the base plate 110 for each TE leg shown in FIG. 3. Thestress mapping data is taken for a 4-leg unit of FIG. 3 with insertedmetal cap for each TE leg and the whole unit being held at thesubstantially the same operation environment applied to the 4-leg unitshown in FIG. 2. As shown, the thermally induced stress now is muchsmaller across the whole unit as indicated by more than 10 folds lowervon Mises stress, and Gauss-point evaluation in the 16 MPa range whencompared with that in FIG. 2 for the unit without any specific CTEmatching. The stress distribution is also much more even through thelength of the TE leg and near the junction region near the metal pad 111and the first end region 101. The relatively high stress leveloriginally appeared in four lower corner edges as shown in FIG. 2 now isnegligible, almost disappears.

Note that in some embodiments, the ductile cap 140 can be made with aslightly bigger base area in contact with the metal pad below than thatnear the first end region of each TE leg. During the operation, theductile cap is allowed to expand more near the metal pad butsubstantially without causing significant stresses to the upper regionin contact with the TE leg, thereby reducing the mismatching stress inthe TE leg. As shown in the top region with shunt plate in FIG. 4D,although somewhat larger stress would appear to be illustrated, notethat this is merely due to the whole scale of the measurement is reducedby more than 10 fold relative to that in FIG. 2 so that the sensitivityin FIG. 4D is higher. On other hand, the flexible shunt plate indeedplays its role to help reducing the stress in the TE leg induced fromCTE mismatch from the cold-side junction during operation.

In an embodiment, the presence of a cap, e.g., a ductile metal cap, onthe cold side of a thermoelectric material can or will increase thereliability of solder joint (for bonding the shunt plate with the TElegs) by reducing the stresses induced in the joint. In anotherembodiment, the presence of one or more caps, e.g., ductile metal caps,each with Ti/Ni metallization can or will provide increased bondstrength and improved adhesion during co-sintering process of the CTEmatching material with the TE materials. This may also provide increasedleg material strength during a dicing process from a bigger sinteredbulk of TE material to produce each individual TE leg. In yet anotherembodiment, a co-sintering process to form a TE leg with CTE matchingend cap in one step can reduce the total number of processes forfabricating and packaging a thermoelectric device. This allowselimination of one or more metallization process for forming TE legs,therefore reducing cost by simplifying manufacturing process of thethermoelectric devices. Of course, there are many other variations,alternatives, and modifications. Exemplary methods for forming TEmaterials using sintering are described below with reference to FIG. 8,and exemplary methods of forming thermoelectric devices are describedfurther below with reference to FIG. 6.

In another embodiment, a method for thermal stress and coefficient ofthermal expansion (CTE) mismatch management in thermoelectric package isprovided. Reduction of thermal stresses in thermoelectric legs duringtheir manufacturing process and thermoelectric operation can be providedby using one or more CTE matching materials, e.g., ductile materials, atthe interface between the thermoelectric materials and an electricalconnector, e.g., shunt material, which can be disposed on or in contactwith a ceramic base plate. Exemplary advantages of implementing certainembodiments of the present invention can include reducing or minimizingthermal stresses, increasing bond strength between TE material andpackage, and reducing manufacture cost of thermoelectric package bysimplifying packaging steps and facilitating brazing steps.

Thermoelectric device 2000 illustrated in FIG. 3 can be configured topump heat from shunt plate 120 to base plate 110 through thermoelectriclegs 100 based on a voltage applied between those thermoelectric legsvia respective metal pads 111. As such, thermoelectric device 2000 canbe used to cool shunt plate 120 or a body coupled thereto and to heatbase plate 110 or a body coupled thereto, or to heat shunt plate 120 ora body coupled thereto and to cool base plate 110 or a body coupledthereto. For example, the second end region 102 of each thermoelectricleg 100 can be in thermal and electrical contact with shunt plate 120from which heat is to be pumped, and the first end region 101 of eachthermoelectric leg 100 can be in thermal and electrical contact withbase plate 110 to which heat is to be pumped. Accordingly, one or moreof thermoelectric legs 100 can be configured electrically in series withone another, and thermally in parallel with one another between shuntplate 120 from which heat is to be pumped, and base plate 110 to whichheat is to be pumped. Thermoelectric legs 100 each can include a p-typeor n-type material and first and second caps disposed on either side ofthe p-type or n-type material using any suitable configuration or methodprovided herein. The thermoelectric legs 100 of device 2000 need notnecessarily include the same p-type or n-type material as one another.For example, two of the thermoelectric legs 100 of device 2000 caninclude a p-type material, e.g., Mg₂Si, and two of the thermoelectriclegs 100 of device 2000 can include an n-type material, e.g., MnSi_(x).

A first metal pad 111 of a thermoelectric leg 100 including an n-typematerial can be coupled to any suitable electrical component, and asecond metal pad 111 of a thermoelectric leg 100 including a p-typematerial also can be coupled to that electrical component. Responsive toa temperature differential or gradient between shunt plate 120 and baseplate 110, electrons (e−) flow from the shunt plate 120 to the firstmetal pad 111 through the n-type material, and holes (h+) flow from theshunt plate to the second metal pad 111 through the p-type material,thus generating a current. An electrical potential or voltage betweenthe first and second metal pads 111 is created by having eachthermoelectric leg 100 in a temperature gradient with electric currentflow created as the p-type and n-type materials are connected togetherelectrically in series and thermally in parallel. The current generatedby device 2000 can be utilized in any suitable manner. For example, thefirst metal pad 111 can be coupled to an anode 28 via a suitableconnection, e.g., an electrical conductor, and the second metal pad 111can be coupled to a cathode via a suitable connection, e.g., anelectrical conductor. The anode and cathode can be connected to anysuitable electrical device so as to provide a voltage potential orcurrent to such device. Exemplary electrical devices include batteries,capacitors, motors, resistors, and the like. For example, the anode andcathode respectively can be coupled to first and second terminals of aresistor. The resistor can be a stand-alone device or can be a portionof another electrical device to which the anode and cathode can becoupled. Alternatively, the first metal pad 111 can be coupled to thecathode of a battery or other power supply, and the second metal pad 111can be coupled to the anode of that battery or other power supply, andresponsive to a voltage applied by the battery or other power supplybetween the first and second metal pads, electrons (e−) flow from shuntplate 120 to the first metal pad 111 through the n-type material, andholes (h+) flow from shunt plate 120 to the second metal pad 111 throughthe p-type material, thus pumping heat from shunt plate 120 to baseplate 110. The pumping of heat from shunt plate 120 to base plate 110suitably can be used to cool shunt plate 120. For example, shunt plate120 can be coupled to a computer chip or other electrical component thatcan benefit from thermoelectric cooling.

As noted further above, FIGS. 4A-4D respectively show exemplary resultsof 3D stress-level mapping for different configurations of structure2000 illustrated in FIG. 3. More specifically, FIGS. 4A-4D show an FEAanalysis respective for four different multi-leg TE packages includingtwo kinds of TE legs without metal caps and two other kinds of TE legswith metal caps according to embodiments of the present invention. Theresults show the benefit of reduced thermally-induced stress from theuse of metal caps, with either a silicon nitride or alumina substrate,when compared to the thermally-induced stress of TE legs that do nothave metal caps. The thermal stress is reduced from 600 MPa to 16 MPa.In illustrative embodiments, the CTE of the metal caps can either be anintermediate value between that of the TE material and that of shuntmaterial to have a gradual change in CTE difference or is closelymatched to the TE material, such that the thermal stress is mostlysustained by the strong metal cap rather than imposed onto the weak TEleg. Exemplary parameters used in preparing FIGS. 4A-4D, and exemplaryresults of FIGS. 4A-4D, are summarized in Table 1.

TABLE 1 Exemplary Parameters and Results Parameter FIG. 4A FIG. 4B FIG.4C FIG. 4D Design Base case Alumina Silicon Alumina configuration(silicon base plate nitride base base plate nitride base (no metal plateand and TE plate, no caps) TE legs with legs with metal caps) metal capsmetal caps Base plate Silicon Alumina Silicon Alumina material nitridenitride Hot side Incusil ABA Incusil ABA Incusil ABA Incusil ABA bonding(brazing) (brazing) (brazing) (brazing) material TE material MagnesiumMagnesium Magnesium Magnesium for n-type leg silicide silicide silicidesilicide Cap material N/A N/A Nickel Nickel for n-type leg TE materialManganese Manganese Manganese Manganese for p-type leg silicide silicidesilicide silicide Cap material N/A N/A Chromium Chromium for p-type legCold side 94Sn—4Ag 94Sn—4Ag 94Sn—4Ag 94Sn—4Ag bonding (solder) (solder)(solder) (solder) material Lead frame Copper Copper Copper Copper (metalcold- side structure with tabs that can be removed) Hot side 388° C.388° C. 388° C. 388° C. temperature (boundary condition) Cold side 172°C. 172° C. 172° C. 172° C. temperature (boundary condition) FixedShorter edge Shorter edge Shorter edge Shorter edge constraint of baseplate of base plate of base plate of base plate Model used Elasto-Elasto- Elasto- Elasto- for hot side plastic plastic plastic plasticbonding material Model used Viscoplastic Viscoplastic ViscoplasticViscoplastic for cold side bonding material Maximum 600 MPa 600 MPa 16MPa 16 MPa Von Mises Stress (Exemplary Result)

In another embodiment, the present invention provides a compositestructure of a bulk-size thermoelectric material sandwiched by tworelatively thick cap layers, e.g., metal layers. In a specificembodiment, the sandwich composite structure is, or includes, at least aportion of a pellet made by co-sintering powdered thermoelectricmaterial and powdered cap material(s), e.g., metal material(s). However,it should be appreciated that such sandwich composite structures can beformed using other suitable methods.

In some embodiments, the thickness of the TE material can beapproximately 0.5 mm to approximately 20 mm. For example, the thicknessof the TE material can be approximately 1 mm to approximately 10 mm. Or,for example, the thickness of the TE material can be approximately 2 mmto approximately 5 mm. Or, for example, the thickness of the TE materialcan be approximately 2 mm to approximately 5 mm. Or, for example, thethickness of the TE material can be approximately 2 mm to approximately3 mm. Additionally, or alternatively, the thickness of each cap can begreater than approximately 0.2 mm. For example, the thickness of eachcap can be greater than approximately 0.3 mm. Or, for example, thethickness of each cap can be approximately 0.2 mm to approximately 2 mm.Or, for example, the thickness of each cap can be approximately 1 mm toapproximately 2 mm. Or, for example, the thickness of each cap can beapproximately 0.5 mm to approximately 1.5 mm. For example, the thicknessof each cap can be greater than approximately 2 mm. Additionally, oralternatively, the thickness of the p-type or n-type material can beapproximately 0.2 mm to approximately 5 mm. For example, the thicknessof the p-type or n-type material can be approximately 0.5 mm toapproximately 4 mm. Or, for example, the thickness of the p-type orn-type material can be approximately 0.5 mm to approximately 2.5 mm. Or,for example, the thickness of the p-type or n-type material can beapproximately 0.5 mm to approximately 2 mm. As used herein, the terms“about” and “approximately” are intended to mean within plus or minus10% of the stated value.

As illustrative examples, FIG. 5A and FIG. 5B are exemplary images ofco-sintered bulk thermoelectric composite sandwich materials (which alsomay be referred to as “pellets”) that have been diced into multiplepieces according to embodiments of the present invention. These diagramsare merely examples, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. In FIG. 5A, on the left,the central material is magnesium silicide (Mg₂Si), while the materialsat top and bottom are nickel caps. In FIG. 5B, on the right, the middlesilver-color material is manganese silicide (MnSi_(1.73)), while the topand bottom layers are chromium caps. The top area on both samplesincludes a thin film of gold deposited after sintering. In anembodiment, as shown in FIGS. 5A and 5B, the pellet formed from theco-sintering process includes two metal caps that have been polished toreduce the thickness to about 0.3 mm from initial thickness of about1-1.25 mm and the thermoelectric material in the middle region remainsto be the original thickness of about 2.5-3.0 mm. There can be about 10%variation in those thickness values for a random just-sintered pellet.Pellets can be polished to reduce the metal cap thickness tospecification, as well as to achieve planar parallel surfaces, beforedicing into individual thermoelectric legs with an exemplary base of 2mm×2 mm square, though other shapes and sizes are possible, e.g.,pellets or diced thermoelectric legs with a cross-sectional area in therange of approximately 1.8 mm×1.8 mm to approximately 3.6 mm×1.8 mm.Exemplary methods for forming TE materials using sintering are describedbelow with reference to FIG. 8, and exemplary methods of formingthermoelectric devices are described further below with reference toFIG. 6.

In one illustrative embodiment, when making the cap, e.g., metal cap,using the co-sintering method of the present invention, adding metalpowder material to above 1 mm in thickness facilitates formation of aflat surface in a pre-pressing step such that the flat surface issubstantially not slanted and has full or substantially full coverage ofthe underneath material. Without wishing to be bound by any theory, itis believed that this feature further can lead to an improvedmetal-thermoelectric interface structure formed during the co-sinteringprocess that is useful, or potentially even crucial, for the formationof bulk thermoelectric composite sandwich structure substantially freefrom cracking and free from delamination of the metal cap from thesintered thermoelectric layer during the formation of individual legs bydicing.

Another manifestation of certain embodiments of the present invention isto use a thin metal layer in between two thicker layers ofthermoelectric material to bond them together using the co-sinteringprocess, allowing formation of a thicker bulk thermoelectric. Yetanother embodiment is to use a very thin metal cap as a conductiveinterface for brazing together with conductive shunts.

Another optional process that could be used in place of a spark-plasmasintering (SPS) based sintering method such as described further hereinis hot pressing wherein the high temperature is achieved with inductionor indirect resistance heating while pressure applied to the sample canbe much higher than that used in SPS sintering. Depending on thematerial selection for the thermoelectric powder material and metalpowder material, the specific process condition may be different.

As noted further above, many benefits are provided upon the applicationof various embodiments of the present invention. One of them is theability to control thermoelectric property variations out of thesintered thermoelectric composite material. Formation of bulkthermoelectric composite materials using many methods, including theco-sintering process described in certain embodiments of the presentinvention, often yields properties that vary with pellet size due todefects, agglomerates, presence of various material phases, variationsof component concentrations, and other stochastic phenomena. Suchproperty variations can cause nontrivial variation among many small legsthat are cut from a single pellet formed after co-sintering process,resulting in deviation of properties from the design point. As a resultof the thermoelectric material variation, while pellet-levelmeasurements show that electrical resistance of all the diced legs wouldbe expected to fall between 2 and 10 mOhm, the measurement data showedthat resistance for individual legs ranged from 2 to well over 20 mOhm,with outliers even at one order of magnitude higher. In someembodiments, the diced legs characterized by an electrical resistancebetween 2 and 10 mOhm can include a cross-sectional area in the range ofapproximately 1.8 mm×1.8 mm and approximately 3.6 mm×1.8 mm and athickness of the p-type or n-type material in the range of approximately0.5 mm to approximately 2 mm. It should be appreciated that othersuitable ranges of cross-sectional area and other thicknesses of p-typeor n-type materials can be selected so as to provide thermoelectricmaterials or diced legs characterized by an electrical resistancebetween 2 and 10 mOhm.

FIG. 6 illustrates steps in an exemplary method for forming athermoelectric device according to certain embodiments of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. For example,as illustrated in method 600 of FIG. 6 a thermoelectric material can beprovided that includes a p-type or n-type material and first and secondcaps respectively including first and second cap materials respectivelydisposed on either side of the p-type or n-type material (601).Exemplary, nonlimiting materials suitable for use in such p-type orn-type materials or in such first and second cap materials are describedelsewhere herein. Additionally, exemplary, nonlimiting methods forforming such thermoelectric materials are described further herein,e.g., with reference to FIG. 8. In certain embodiments, the first andsecond cap materials each can respectively include an independentlyselected transition metal, and a thickness of each of the first andsecond caps can be approximately 0.2 mm to approximately 2 mm, althoughother thicknesses mentioned herein suitably can be used. Thethermoelectric material can be diced to form a plurality of individualthermoelectric legs (602). Methods of dicing thermoelectric materialsare known in the art, and include wire saw, blade dicing, or anystandard process used in the semiconductor industry. The electricalresistance, or any other appropriate thermal, mechanical, or electricalcharacteristic, of each of the individual thermoelectric legs can betested (603). Exemplary thermal characteristics that can be testedinclude thermal resistance. Exemplary electrical characteristics thatcan be tested include electrical resistance and electrical power. Theindividual thermoelectric legs can be sorted based on the testedelectrical resistance or other characteristic (604). In one nonlimitingexample, each individual leg can be sorted based on whether the measuredelectrical resistance or other characteristic varies from the targetvalue by 10% or less, or by 10-20%, or by 20-30%, or by 30-40%, or by40-50%, or by greater than 50%, and so on. Any suitable criterion can beused to sort the individual thermoelectric legs. At least one of thefirst and second caps of at least one of the sorted individualthermoelectric legs can be coupled to an electrical connector (605). Forexample, a given thermoelectric leg may meet an electrical resistancecriterion for use in a particular application, and thus can be coupledto an electrical connector in accordance with that application.

FIG. 7 is a plot illustrating the number of useful thermoelectric legsformed by dicing a thermoelectric material according to certainembodiments of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. For example, FIG. 7 shows the percentage of the dicedlegs from over 160 pellets that were deemed to “pass” the resistancecriterion, even though the pellets themselves prior to dicing all passthe manufacturing specification for resistance. The resistance criterionwas an electrical resistance of 2-10 mOhm. In this example, the dicedlegs had a cross-sectional area of approximately 1.8 mm by 1.8 mm. Thecaps included nickel, and the thicknesses of the caps varied betweenabout 500 μm and about 2 mm. The thermoelectric material included Mg₂Si,and the thickness of the thermoelectric material varied between about1.5 and 2.5 mm. The average particle size of Mg₂Si powder used toprepare the pellets was not specifically measured, but was believed tobe in the range of approximately 1 μm and 10 μm. Spark plasma sintering(SPS) was used to prepare the pellets in a manner consistent with thatdescribed below with reference to FIG. 8, and a wire saw was used todice the pellets. A homemade system was used to measure resistance, thatincluded Kelvin probes and a Keithley nanovoltmeter and current source.As used herein, “average particle size” is intended to mean at least oneof the arithmetic mean of the particle sizes, the median particle size,or the mode of the particle size. Illustratively, “average particlesize” can mean the median particle size.

Note that pellets that were otherwise the same but formed using twodifferent average particle sizes of nickel powder were used to preparediced legs that were measured in association with FIG. 7. Morespecifically, a first set of legs referred to in FIG. 7 as “Using largeNi” were prepared by dicing pellets of a thermoelectric materialincluding first and second caps formed using nickel powder having, orbeing characterized by, an average particle size of 150 μm. A second setof legs referred to in FIG. 7 as “Using small Ni” were prepared bydicing pellets of a thermoelectric material including first and secondcaps formed using commercially purchased nickel powder having, or beingcharacterized by, an average particle size of 5 μm or less. It readilycan be understood from FIG. 7 that the percentage of passing legs in onepellet was found to be significantly higher for pellets formed using thenickel powder having, or being characterized by, an average particlesize of 5 μm or less, as compared to pellets formed nickel powderhaving, or being characterized by, an average particle size of 150 μm.For example, the data on the far right of FIG. 7 indicate that about 45pellets formed using the nickel powder having, or being characterizedby, an average particle size of 5 μm or less each yielded 91% to 100% ofthermoelectric legs that met the resistance criterion diced from thosepellets, and that about 5 pellets formed using the nickel powder having,or being characterized by, an average particle size of 150 μm eachyielded 91% to 100% of thermoelectric legs that met the resistancecriterion diced from those pellets.

Moving leftward across FIG. 7, about 16 pellets formed using the nickelpowder having, or being characterized by, an average particle size of 5μm or less each yielded 81% to 90% of thermoelectric legs that met theresistance criterion diced from those pellets, and that about 2 pelletsformed using the nickel powder having, or being characterized by, anaverage particle size of 150 μm each yielded 81% to 90% ofthermoelectric legs that met the resistance criterion diced from thosepellets. Continuing to move leftward across FIG. 7, about 15 pelletsformed using the nickel powder having, or being characterized by, anaverage particle size of 5 μm or less each yielded 71% to 80% ofthermoelectric legs that met the resistance criterion diced from thosepellets, and that about 6 pellets formed using the nickel powder having,or being characterized by, an average particle size of 150 μm eachyielded 71% to 80% of thermoelectric legs that met the resistancecriterion diced from those pellets. Continuing to move leftward acrossFIG. 7, about 8 pellets formed using the nickel powder having, or beingcharacterized by, an average particle size of 5 μm or less each yielded61% to 70% of thermoelectric legs that met the resistance criteriondiced from those pellets, and that about 3 pellets formed using thenickel powder having, or being characterized by, an average particlesize of 150 μm each yielded 61% to 70% of thermoelectric legs that metthe resistance criterion diced from those pellets. Continuing to moveleftward across FIG. 7, about 6 pellets formed using the nickel powderhaving, or being characterized by, an average particle size of 5 μm orless each yielded 51% to 60% of thermoelectric legs that met theresistance criterion diced from those pellets, and that about 2 pelletsformed using the nickel powder having, or being characterized by, anaverage particle size of 150 μm each yielded 61% to 70% ofthermoelectric legs that met the resistance criterion diced from thosepellets. Continuing to move leftward across FIG. 7, about 2 pelletsformed using the nickel powder having, or being characterized by, anaverage particle size of 5 μm or less each yielded 41% to 50% ofthermoelectric legs that met the resistance criterion diced from thosepellets, and that about 1 pellet formed using the nickel powder having,or being characterized by, an average particle size of 150 μm yielded41% to 50% of thermoelectric legs that met the resistance criteriondiced from those pellets. Continuing to move leftward across FIG. 7,about 4 pellets formed using the nickel powder having, or beingcharacterized by, an average particle size of 5 μm or less each yielded31% to 40% of thermoelectric legs that met the resistance criteriondiced from those pellets, and that about 0 pellets formed using thenickel powder having, or being characterized by, an average particlesize of 150 μm each yielded 31% to 40% of thermoelectric legs that metthe resistance criterion diced from those pellets. Continuing to moveleftward across FIG. 7, about 1 pellet formed using the nickel powderhaving, or being characterized by, an average particle size of 5 μm orless yielded 21% to 30% of thermoelectric legs that met the resistancecriterion diced from those pellets, and that about 1 pellet formed usingthe nickel powder having, or being characterized by, an average particlesize of 150 μm yielded 21% to 30% of thermoelectric legs that met theresistance criterion diced from those pellets. Continuing to moveleftward across FIG. 7, about 0 pellets formed using the nickel powderhaving, or being characterized by, an average particle size of 5 μm orless each yielded 11% to 20% of thermoelectric legs that met theresistance criterion diced from those pellets, and that about 2 pelletsformed using the nickel powder having, or being characterized by, anaverage particle size of 150 μm each yielded 11% to 20% ofthermoelectric legs that met the resistance criterion diced from thosepellets. The data on the far left of FIG. 7 indicate that about 1 pelletformed using the nickel powder having, or being characterized by, anaverage particle size of 5 μm or less each yielded 1% to 10% ofthermoelectric legs that met the resistance criterion diced from thosepellets, and that about 10 pellets formed using the nickel powderhaving, or being characterized by, an average particle size of 150 μmeach yielded 1% to 10% of thermoelectric legs that met the resistancecriterion diced from those pellets.

Accordingly, without wishing to be bound by any theory, it is believedthat forming caps using precursor particle sizes that are relativelysmall, or relatively similar to the particle sizes of precursorparticles for the p-type or n-type material, can significantly improvethe percentage of thermoelectric legs that met the resistance criteriondiced from pellets formed using those particles. For example, athermoelectric device can include a thermoelectric material formed byco-sintering a powder precursor of a first cap material, a powderprecursor of a p-type or n-type material, and a powder precursor of asecond cap material in a sintering die, wherein a particle size ratio ofthe powder precursor of the p-type or n-type material to the powderprecursors of the first and second cap materials can be in the range ofapproximately 50:1 to approximately 1:50, or approximately 50:1 to 1:1,or approximately 1:50 to 1:1. For example, a particle size ratio of thepowder precursor of the p-type or n-type material to the powderprecursors of the first and second cap materials can be in the range ofapproximately 20:1 to approximately 1:20, e.g., can be approximately1:20, or can be in the range of approximately 10:1 to approximately1:10, or can be in the range of approximately 5:1 to approximately 1:5,or can be in the range of approximately 4.4:1 to approximately 1:3.4. Incertain embodiments, the powder precursors of one or both of the firstand second cap materials can have a particle size of about 100 nm toabout 150 μm, e.g., about 10 μm to about 150 μm, or about 100 nm toabout 10 μm, e.g., about 5 μm or less. Additionally, or alternatively,in certain embodiments and in any suitable combination with any of theratios provided herein (or any other ratio) or any of the cap materialparticle sizes provided herein (or any other cap material particlesizes), the powder precursor of the p-type or n-type material can have aparticle size in the range of about 10 nm to about 100 μm, e.g., 10 nmto about 1 μm, e.g., about 100 nm, or, e.g., about 44 μm. Someexemplary, nonlimiting combinations of particle sizes are described ingreater detail herein.

Alternatively, or additionally, without wishing to be bound by anytheory, it is believed that a capability of sorting diced legs isuseful, and potentially can even be critical for certain applications,in order to discard the numerous “failing” legs that do not pass theresistance criterion. Metal caps can be formed according to certainembodiments of the present invention that are thick enough to allow thecurrent spreading necessary to perform consistent, accurate, andrepeatable through-plane resistance measurements, which makes itpossible to measure electrical properties on individual leg-level sothat the deviant or poorer performing leg samples can be discarded, andthe top or better performing leg performers can be selectively placed inthe most critical locations for assembling a thermoelectric system withrelatively high efficiency and improved or optimal performance. Withoutwishing to be bound by any theory, such a measurement is not believed tobe possible with only thin-film electrical contacts for reasons such asmentioned further elsewhere herein. It should be understood that the useof relatively thick metal caps can be, but need not necessarily, becombined with the use of certain particle sizes to form the p-type orn-type material and first and second caps.

Other potential exemplary benefits of certain embodiments of the presentinvention lie in the enhancement of mechanical robustness of individualthermoelectric legs diced from the co-sintered pellet with themetal-capped thermoelectric composite structure. Without wishing to bebound by any theory, it is believed that relatively thick metal caps(e.g., caps thicker than about 0.2 mm) formed according to someembodiments of the present invention can enable the thermoelectricmaterial to successfully withstand dicing into smaller pieces ascompared to relatively thin caps, especially as a thicker thermoelectricmaterial layer may be desired. It is believed that without themechanical reinforcement of the metal caps with a thickness greater than0.2 mm, e.g., substantially greater than 0.2 mm, or substantiallygreater than 0.3 mm, the thermoelectric material can tend to crumbleupon removal from the sintering dies or during dicing. Typicalthermoelectric materials can have a much lower thermal expansioncoefficient than the metal materials that serve as their electrodes.Thus it is common in such typical materials to have problems withdelamination or cracking as these two dissimilar CTE materials arejoined together. The co-sintered metal caps that can be formed accordingto some embodiments of the present invention can help to reduce or avoidthis problem as the materials including their forms (e.g., powderparticle size) can be chosen by design to have an intermediate thermalexpansion coefficient, thus easing the transition between materials.

Alternate, or additional, benefits of certain embodiments of the presentinvention also lie in the enhancement of electrical robustness ofindividual thermoelectric legs diced from the co-sintered pellet withthe metal-capped thermoelectric composite structure. Unlike manyconventional cases of using a nanometer thick metal film at the bothends of thermoelectric material, where current has limited ability tospread across the entire sample, the metal caps co-sintered with thethermoelectric composite material can be many tens to hundreds ofmicrons thick which allows current to more freely spread. The metal capsaccording to some embodiments of the present invention can help toprovide a means of easily coupling, e.g., soldering or brazing, thethermoelectric legs to electrical connectors such as electrical leadsand module shunts, for making the overall thermoelectric system and alsocan serve as a diffusion barrier preventing unwanted material migrationduring system operation. Co-sintered metal caps also can adhererelatively well to the thermoelectric material, thus reducing orpreventing failure at the bond interface and resulting in a relativelylow parasitic resistance due to similar properties to dopedsemiconductor impurity.

For example, Ni powder and Mg₂Si powder are an exemplary, relativelygood metal-thermoelectric material pair for forming an n-typethermoelectric leg with conductor caps using co-sintering such asdescribed further herein with reference to FIG. 8. As another example,Cr powder and MnSi_(1.73) powder have been shown to be another goodmetal-thermoelectric material pair for forming a p-type thermoelectricleg with conductor caps using co-sintering such as described furtherherein with reference to FIG. 8. Other TE material-metal combinationscan also be used. Both p- and n-types of thermoelectric legs can beformed using the method described in some embodiments of the presentinvention and can be conveniently sorted out and selectively placed todesired positions for assembling a high performance thermoelectricmodule. Therefore, certain embodiments of the present methods can berelatively, or very, robust in making and sorting both the n-type andp-type thermoelectric legs such as to substantially improve themanufacturability of the thermoelectric power generation system. In aspecific embodiment, using co-sintered metal caps eliminates at leastone metallization process step by combining the metallization with theconsolidation of the thermoelectric material.

In another embodiment, bulk-size metal-capped thermoelectric compositesandwich structure and methods for forming the same are provided. Merelyby way of example, the invention has been applied to co-sinter metalpowders and thermoelectric composite powders with selected materialtype, mass, particle size for forming a metal-capped thermoelectriccomposite pellet that can be diced into multiple thermoelectric legswithout mechanical cracking or delamination of the metal cap and furtheris capable of easily sorting electrical, e.g., resistance, property ofindividual thermoelectric legs for the manufacture of thermoelectricsystems.

FIG. 8 illustrates steps in an exemplary method for forming athermoelectric material, according to certain embodiments of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. Method 800illustrated in FIG. 8 includes loading a powder precursor of a first capmaterial into a sintering die, and assembling one or more punches to thepowder precursor of the first cap material in the sintering die (801).For example, the powder precursor of the first cap material may include,or may consist essentially of, the elements of the first cap material.Exemplary cap materials and particle sizes are described elsewhereherein. In one illustrative, nonlimiting embodiment, the first capmaterial includes at least one transition metal, and the powderprecursor of the first cap material includes a powder including the atleast one transition metal. For example, the powder precursor of thefirst cap material can include, or can consist essentially of, ametallic powder. Exemplary metallic powders include nickel powder andchromium powder, although other non-limiting, exemplary metals andmaterials suitable for use in preparing a first cap material arementioned elsewhere herein. Powders including at least one transitionmetal can be commercially purchased or can be prepared using mechanicalball milling or other suitable technique known in the art. In oneillustrative embodiment, the sintering die can include a graphitematerial having, or being characterized by, a size of 20 mm or greater.In one illustrative embodiment, the sintering die can include one ormore walls that define an aperture into which a first punch can beinserted, the powder precursor of the first cap material can be loadedonto the first punch, and a second punch can be loaded onto the powderprecursor. In one example, the aperture is substantially cylindrical. Inanother example, the aperture is substantially square. In yet anotherexample, the aperture is substantially rectangular. Other apertureshapes suitably can be used.

Method 800 illustrated in FIG. 8 also includes applying a first pre-loadvia the one or more punches to the powder precursor of the first capmaterial so as to form a first pre-pressed structure including a firstflat or substantially flat surface (802). In some embodiments, the firstpre-load can include a pressure in the range of approximately 2 MPa toapproximately 80 MPa, e.g., in the range of approximately 10 MPa toapproximately 50 MPa, such as approximately 15 MPa in one illustrativeembodiment. It should be appreciated that applying the first pre-load tothe powder precursor of the first cap material potentially can increasethe temperature of the powder precursor of the first cap material.Additionally, or alternatively, the temperature can be, but need notnecessarily be, varied during step 802, e.g., by heating the sinteringdie, such as by applying a current to the sintering die. Continuing withthe above example in which the sintering die includes one or more wallsthat define an aperture into which a first punch can be inserted, thepowder precursor of the first cap material can be loaded onto the firstpunch, and a second punch can be loaded onto the powder precursor, thefirst pre-load can be applied to the powder precursor of the first capmaterial via the first and second punches so as to form the firstpre-pressed structure including a first substantially flat surface.

Method 800 illustrated in FIG. 8 also includes removing a first punch ofthe one or more punches so as to expose the first substantially flatsurface; loading a powder precursor of a p-type or n-type material intothe sintering die and onto the exposed first flat surface, andassembling the first punch to the powder precursor of the p-type orn-type material in the sintering die (803). For example, the powderprecursor of the p-type or n-type material may include, or may consistessentially of, the elements of the p-type or n-type material. Exemplaryp-type or n-type materials and particle sizes are described elsewhereherein. In one illustrative, nonlimiting embodiment, the p-type orn-type material includes a silicide, and the powder precursor of thep-type or n-type material includes a powder including the silicide. Forexample, the powder precursor of the p-type or n-type material caninclude, or can consist essentially of, magnesium silicide or manganesesilicide, although other non-limiting, exemplary p-type or n-typematerials suitable for use in preparing a p-type or n-type material arementioned elsewhere herein. Powders including a p-type or n-typematerial can be commercially purchased or can be prepared usingmechanical ball milling or other suitable technique known in the art.Continuing with the above example in which the sintering die includesone or more walls that define an aperture into which a first punch canbe inserted, the powder precursor of the material can be loaded onto thefirst punch, and a second punch can be loaded onto the powder precursor,the second punch can be removed so as to expose the first flat surfaceof the first pre-pressed structure, the powder precursor of the p-typeor n-type material can be disposed on the first flat surface, and thesecond punch then replaced.

Method 800 illustrated in FIG. 8 also includes applying a secondpre-load via the one or more punches to the first pre-pressed structureand the powder precursor of the p-type or n-type material so as to forma second pre-pressed structure including a second flat or substantiallyflat surface (804). In some embodiments, the second pre-load can includea pressure in the range of approximately 2 MPa to approximately 80 MPa,e.g., in the range of approximately 10 MPa to approximately 50 MPa, suchas approximately 15 MPa in one illustrative embodiment. It should beappreciated that applying the second pre-load to the first pre-pressedstructure and the powder precursor of the p-type or n-type materialpotentially can increase the temperature of the first pre-pressedstructure or of the powder precursor of the p-type or n-type material,or both. Additionally, or alternatively, the temperature can be, butneed not necessarily be, varied during step 804, e.g., by heating thesintering die, such as by applying a current to the sintering die.Continuing with the above example, the second pre-load can be applied tothe first pre-pressed structure and the powder precursor of the p-typeor n-type material via the first and second punches so as to form thesecond pre-pressed structure including a second substantially flatsurface.

Method 800 illustrated in FIG. 8 also includes removing the first punchso as to expose the second substantially flat surface; loading a powderprecursor of a second cap material into the sintering die and onto theexposed second flat surface, and assembling the first punch to thepowder precursor of the second cap material in the sintering die (805).For example, the powder precursor of the second cap material mayinclude, or may consist essentially of, the elements of the second capmaterial. Exemplary cap materials and particle sizes are describedelsewhere herein. In one illustrative, nonlimiting embodiment, thesecond cap material includes at least one transition metal, and thepowder precursor of the second cap material includes a powder includingthe at least one transition metal. For example, the powder precursor ofthe second cap material can include, or can consist essentially of, ametallic powder. Exemplary metallic powders include nickel powder andchromium powder, although other non-limiting, exemplary metals andmaterials suitable for use in preparing a second cap material arementioned elsewhere herein. The first and second cap materials caninclude, but need not necessarily include, the same materials as oneanother. Powders including at least one transition metal can becommercially purchased or can be prepared using mechanical ball millingor other suitable technique known in the art. Continuing with the aboveexample, the second punch can be removed so as to expose the second flatsurface of the second pre-pressed structure, the powder precursor of thep-type or n-type material can be disposed on the second flat surface,and the second punch then replaced.

Method 800 illustrated in FIG. 8 also includes applying a third pre-loadvia the one or more punches to the second pre-pressed structure and thepowder precursor of the second cap material so as to form a thirdpre-pressed structure including a third flat or substantially flatsurface (806). In some embodiments, the third pre-load can include apressure in the range of approximately 2 MPa to approximately 80 MPa,e.g., in the range of approximately 10 MPa to approximately 50 MPa, suchas approximately 15 MPa in one illustrative embodiment. It should beappreciated that applying the third pre-load to the second pre-pressedstructure and to the powder precursor of the second cap materialpotentially can increase the temperature of the second pre-pressedstructure or of the powder precursor of the second cap material, orboth. Additionally, or alternatively, the temperature can be, but neednot necessarily be, varied during step 806, e.g., by heating thesintering die, such as by applying a current to the sintering die.Continuing with the above example the third pre-load can be applied tothe second pre-pressed structure and to the powder precursor of thesecond cap material via the first and second punches so as to form thethird pre-pressed structure.

Method 800 illustrated in FIG. 8 also includes sintering the thirdpre-pressed structure so as to form the thermoelectric material (807).During such sintering, a suitable combination of temperature andpressure can be used so as to cause the materials of the thirdpre-pressed structure to fuse with one another. For example, the powderprecursor of the first cap material can fuse so as to form the first capmaterial; the powder precursor of the p-type or n-type material can fuseso as to form the p-type or n-type material; and the powder precursor ofthe second cap material can fuse so as to form the second cap material.Additionally, the first cap material and the p-type or n-type materialcan fuse to one another so as to define a first interface, and thesecond cap material and the p-type or n-type material can fuse to oneanother so as to define a second interface. Exemplary ranges ofpressures that can be applied to the third pre-pressed structure duringsuch sintering can include approximately 2 MPa to approximately 80 MPa,e.g., in the range of approximately 30 MPa to approximately 60 MPa, suchas approximately 60 MPa in one illustrative embodiment, or such asapproximately 55 MPa in another illustrative embodiment, or such asapproximately 50 MPa in another illustrative embodiment, or such asapproximately 45 MPa in another illustrative embodiment, or such asapproximately 40 MPa in another illustrative embodiment, or such asapproximately 35 MPa in another illustrative embodiment, or such asapproximately 30 MPa in another illustrative embodiment. Exemplaryranges of temperatures that can be applied to the third pre-pressedstructure during such sintering can include approximately 300° C. toapproximately 1000° C., e.g., in the range of approximately 300° C. toapproximately 500° C., such as approximately 300° C. in one illustrativeembodiment or approximately 500° C. in another illustrative embodiment,or in the range of approximately 500° C. to approximately 1000° C., suchas in the range of approximately 800° C. to approximately 950° C., suchas approximately 850° C. in one illustrative embodiment, or such asapproximately 900° C. in another illustrative embodiment, or such asapproximately 950° C. in yet another illustrative embodiment. It shouldbe appreciated that any suitable combination of one or more temperaturesand one or more pressures can be used.

For example, sintering the third pre-pressed structure can includeapplying a pressure to the third pre-pressed structure while ramping toan intermediate temperature and holding at that temperature for a firstpre-determined period of time, and subsequently ramping to a highertemperature than the intermediate temperature and maintaining thattemperature for a second pre-determined period of time. As anotherexample, sintering the third pre-pressed structure can include rampingto a first intermediate pressure applied to the third pre-pressedstructure; subsequently reducing to a second intermediate pressure;subsequently ramping to an intermediate temperature and holding at thattemperature for a first pre-determined period of time; and subsequentlyramping to a higher temperature than the intermediate temperature andholding at that temperature for a second pre-determined period of time.Optionally, such sintering of the third pre-pressed structure caninclude subsequently to ramping to the intermediate temperature, rampingto a higher pressure than the second intermediate pressure.

Without wishing to be bound by any theory, it is believed that method800 illustrated in FIG. 8, or the use of suitable combinations ofintermediate temperatures or pressures such as mentioned herein, or thecombination of one or more pre-loads and one or more suitableintermediate temperatures or pressures can enhance bonding between thefirst cap material and the p-type or n-type material, as well as bondingbetween the second cap material and the p-type or n-type material; canreduce resistance of the first cap material; can reduce resistance ofthe second cap material; can reduce resistance of the p-type or n-typematerial; can reduce contact resistance between the first cap materialand the p-type or n-type material; can reduce contact resistance betweenthe second cap material and the p-type or n-type material; can enhancemechanical strength of the resultant thermoelectric material; canenhance flatness of the interface between the first cap material and thep-type or n-type material; can enhance flatness of the interface betweenthe second cap material and the p-type or n-type material; can enhancethe percentage or number of thermoelectric legs that meet a performancecriterion, e.g., a resistance criterion, diced from that material; orcan provide any suitable combination of such improvements.

Additionally, note that method 800 illustrated in FIG. 8, or the use ofsuitable combinations of intermediate temperatures or pressures such asmentioned herein, or the combination of one or more pre-loads and one ormore suitable intermediate temperatures or pressures suitably can be,but need not necessarily be, combined with one or more other featuresprovided herein. For example, as noted above, the first or second cap,or both, can be relatively thick, e.g., thicker than about 0.2 mm, so asto improve the ability to measure a performance of the thermoelectricmaterial, e.g., to measure an electrical resistance of thethermoelectric material, among other benefits that such relatively thickcaps can provide. Or, for example, as noted elsewhere herein, theabsolute particle sizes or the particle size ratio, or both, can beselected so as to enhance the percentage or number of thermoelectriclegs that meet a performance criterion, e.g., a resistance criterion,diced from the thermoelectric material. Some non-limiting examples ofparticle sizes and ratios are provided elsewhere herein.

Additionally, it should be appreciated that any suitable sinteringmethod can be used so as to form the thermoelectric material. In oneillustrative example, spark-plasma sintering (SPS) is used to sinter thethird pre-pressed structure, e.g., by applying one or more suitablepressures to the third pre-pressed structure in a sintering die via oneor more punches assembled to the sintering die, and by applying one ormore suitable temperatures to the third pre-pressed structure, e.g., bypulsing a DC current that passes through the third pre-pressedstructure, the sintering die, and the one or more punches. Otherexemplary sintering methods include electric current assisted sinteringsuch as capacitor discharge sintering or resistance sintering (whichalso may be referred to as hot pressing), and pressureless sintering.

Additionally, note that in embodiments that include pre-loading of thefirst cap material, of the p-type or n-type material, or of the secondcap material, or a combination thereof, such preloading can be performedin any suitable order or combination. For example, the pre-loadsrespectively illustrated at steps 802, 804, 806 can be performed in theillustrated sequence, or in any other suitable sequence. For example, afirst pre-load can be applied to the powder precursor of the first capmaterial at any suitable time so as to form a first pre-pressedstructure, in a manner analogous to that described above with referenceto step 802 of method 800 illustrated in FIG. 8. A second pre-load canbe applied to the powder precursor of the p-type or n-type material atany suitable time, and the second pre-load need not necessarily also beapplied to the first pre-pressed structure in addition to the powderprecursor of the p-type or n-type material in a manner such as describedabove with reference to step 804 of method 900 illustrated in FIG. 8.Instead, in certain embodiments, the second pre-load can be appliedsolely to the powder precursor of the p-type or n-type material so as toform a second pre-pressed structure that is discrete and separate fromthe first pre-pressed structure. Additionally, or alternatively, a thirdpre-load can be applied to the powder precursor of the second capmaterial at any suitable time, and the third pre-load need notnecessarily also be applied to the second pre-pressed structure inaddition to the powder precursor of the second cap material in a mannersuch as described above with reference to step 906 illustrated in method800 of FIG. 8. Instead, in certain embodiments, the third pre-load canbe applied solely to the powder precursor of the second cap material soas to form a third pre-pressed structure that is discrete and separatefrom the first or second pre-pressed structure, or both. The first,second, and third pre-pressed structures subsequently can be stacked ina sintering die, e.g., with the second pre-pressed structure stackedupon the first pre-pressed structure, and the third pre-pressedstructure stacked upon the second pre-pressed structure, and the first,second, and third pre-pressed structures then sintered together in amanner analogous to that described above with reference to step 807 ofmethod 800 illustrated in FIG. 8. Alternatively, the first and secondpre-pressed structures can be stacked and sintered together andsubsequently sintered to the third pre-pressed structure, or the secondand third pre-pressed structures can be stacked and sintered togetherand subsequently sintered to the first pre-pressed structure. Otherpermutations suitably can be used.

Some non-limiting, exemplary thermoelectric materials and devices andmethods of forming and using the same now will be described.

In one illustrative example, the thermoelectric material includesmagnesium silicide prepared by sintering magnesium silicide in poweredform and the metal material includes metal prepared by co-sintering ametallic powder with the magnesium silicide in powdered form, e.g.,includes nickel prepared by co-sintering nickel powder with themagnesium silicide in powdered form. In an implementation, the magnesiumsilicide in powered form is synthesized starting from Si and Mgelemental materials using a mechanical alloying ball mill process thatresulted in an ultra fine powder. In one illustrative embodiment, theball milling process is conducted in an argon environment with oxygenconcentration under 200 ppm. Associated with the ball milling process,chemical reactions between Si and Mg powders take place to form Mg₂Si inan ultra fine powder form with an average particle size of about 10 nmto about 100 μm, e.g., 10 nm to about 1 μm, e.g., about 100 nm. In oneillustrative embodiment, the reacted powders are handled in a nitrogenenvironment to prevent oxidation of the Mg₂Si material prior to thesintering process. The metallic powder, e.g., nickel powder, can have aparticle size of about 100 nm to 150 μm, e.g., about 10 μm to about 150μm, or about 100 nm to about 10 μm, e.g., about 5 μm or less.Illustratively, a particle size ratio of the Mg₂Si ultrafine powder tothe metallic powder is in the range of approximately 1:50 toapproximately 50:1, or approximately 50:1 to 1:1, or approximately 1:50to 1:1, e.g., in the range of approximately 20:1 to approximately 1:20,e.g., can be approximately 1:20, or can be in the range of approximately10:1 to approximately 1:10, or can be in the range of approximately 5:1to approximately 1:5.

In another implementation of the material preparation for theco-sintering process, the nickel powder is also handled in an inertenvironment, e.g., including Argon or Nitrogen.

In an alternative illustrative example, the thermoelectric materialincludes manganese silicide. In an implementation of the materialpreparation for the co-sintering process, the manganese silicide is ballmilled in an argon environment to achieve particle size of about 10 nmto about 100 μm, e.g., 10 nm to about 1 μm, e.g., about 44 μm, or, e.g.,smaller than 44 μm. In a specific, non-limiting embodiment, the formedmanganese silicide powder is MnSi_(x) (in one embodiment, x is about1.73) powder. Correspondingly, in one illustrative embodiment, the metallayer precursor material to be co-sintered with the manganese silicidepowder is selected to be chromium powder with a particle size of about100 nm to 150 μm, e.g., about 10 μm to about 150 μm, or about 100 nm toabout 10 μm, e.g., about 5 μm or less. Illustratively, a particle sizeratio of the MnSi_(x) ultrafine powder to the metallic powder is in therange of approximately 1:50 to approximately 50:1, or approximately 50:1to 1:1, or approximately 1:50 to 1:1, e.g., in the range ofapproximately 20:1 to approximately 1:20, e.g., can be approximately1:20, or can be in the range of approximately 10:1 to approximately1:10, or can be in the range of approximately 5:1 to approximately 1:5,or can be in the range of approximately 4.4:1 to approximately 1:3.4.

Some benefits of certain embodiments of the present invention lie inproper selection of relevant materials with optimal powder particlesizes and masses for using co-sintering methods to form high-performancebulk thermoelectric composite material constrained by metal material.For example, in one non-limiting embodiment, to form a bulk (inmillimeter scale) n-type thermoelectric material with two metal caplayers, about 1.4 g of magnesium silicide powder with particle diameterof about 100 nm can be co-sintered with two nickel caps using about 2.75g in each cap and particle size of about 5 μm. In an embodiment, theimpact of the powder particle sizes (of the selected materials) is shownin the measurement result of the contact resistance as well as in thedelamination-free interface of the co-sintered metal-capped bulkthermoelectric composite pellet. The pellet thickness, according tocertain embodiments of the present invention, can be as thin as 2-3 mm,or even thinner such as described elsewhere herein, without cracking. Incertain embodiments, the Ni cap thickness can be 1-2 mm or more inthickness to enhance bonding strength for holding the 20 mm or largersized pellet. When Ni powder with about 150 μm particle size was used,contact resistances between the thermoelectric and the metal cap layeris high (>20 mOhm). Alternatively, when Ni powder with about 5 μm orless particle size was used, contact resistances decreased significantly(smaller than a few mOhm). In one illustrative embodiment, thecross-sectional area of a thermoelectric material or thermoelectric legcharacterized by such contact resistances (e.g., 2-10 mOhm) can be inthe range of approximately 1.8 mm×1.8 mm and approximately 3.6 mm×1.8mm, and the thickness of the p-type or n-type material can be in therange of approximately 0.5 mm to approximately 2.5 mm. Exemplarycomparative results for thermoelectric legs diced from thermoelectricpellets formed using Ni powder with about 150 μm particle size or usingNi powder without about 5 μm or less particle size are described furtherabove with reference to FIG. 7.

As another non-limiting example, a bulk p-type thermoelectric compositematerial can be made by co-sintering about 15.68 g manganese silicide asthe thermoelectric powder (with particle size <44 μm) with two metal caplayers of about 10 g of chromium powder with particle size about 10 μm.FIGS. 9A-9B are scanning electron microscope (SEM) images of thisexemplary thermoelectric material. These diagrams merely examples, whichshould not unduly limit the scope of the claims. One of ordinary skillin the art would recognize many variations, alternatives, andmodifications. Just as with the magnesium silicide and nickel structure,it was found that the contact resistance of the manganese silicide withchromium caps went down significantly when finer chromium powder wasused in the cap layer. In another embodiment, nickel was found not tobond onto manganese silicide, at least under the conditions used toattempt to prepare that particular pellet. The p-type manganese silicidepellet size, thickness, and chromium caps thickness can be also similarto that mentioned for n-type magnesium silicide pellet and nickel caps.Of course, variations and modifications of these parameters are notlimited by the method described here, instead, the method is proven tobe relatively, or very, scalable for making different sizedthermoelectric legs for various applications. Methods such as describedabove with reference to FIG. 8 suitably can be used to sinterthermoelectric materials.

Without wishing to be bound by any theory, it is believed that theparticle size of both the thermoelectric (p-type or n-type) precursorpowder and cap precursor powders, e.g., metallic powders, is believed toplay a useful and, in certain embodiments, an important role in theformation of an interface region between the sintered thermoelectricmaterial in the middle region and two co-sintered metal layers as thetop and bottom caps. For example, without wishing to be bound by anytheory, it is believed that during the co-sintering process, hightemperature and pressure can cause different particles to diffuse intovoid regions between particles and/or form neck-like structures withneighboring particles. Achieving strong interface adhesion and goodelectrical contact between the thermoelectric and metallic materialspotentially can depend, at least in part, on properly choosing the sizeand type of powders for use in co-sintering. Strong adhesion isespecially important for preventing pellet cracking or delamination ofthe metal layer through the dicing process. In some prior works ofco-sintering a thermoelectric material with a metallic layer, theparticle sizes of the thermoelectric material are orders of magnitudebigger, leading to relatively poor adhesion and relatively high contactresistance of the metallic layer with the thermoelectric material (e.g.,Mg₂Si). According to certain embodiments, without wishing to be bound byany theory, it is believed that until the present invention, nosuccessful method had been demonstrated for manufacturing a large pelletof a high performance thermoelectric composite material caped with tworelatively thick metal caps in strong bonding and low contact resistancesuch that the large pellet could be diced into a plurality ofthermoelectric legs without cracking and survive the highthermally-induced stresses of the sintering process.

In an alternative embodiment, the present invention provides anexemplary method for forming a thermoelectric composite sandwichstructure that contains a bulk-size thermoelectric material capped withtwo metal thick layers. The exemplary method uses a co-sintering processto form this thermoelectric composite sandwich structure. The exemplarymethod includes steps of loading powder materials layer-by-layer into asintering die and steps of pre-pressing each powder layer to create flatinterfaces. In a specific embodiment, the exemplary method includesadding metal powders with pre-selected particle size into a sinteringdie. Further, the exemplary method includes a pre-pressing process whichincludes assembling punches to the sintering die associated with a SPS(Spark Plasma Sintering) tool and applying a pre-load of about 15 MPathrough the punches to create a first flat or substantially flat surfacefor the loaded metal powder. Illustratively, the sintering die isprovided by graphite material having, or being characterized by, a sizeof 20 mm or greater for improving the scalability of the manufacture ofthermoelectric legs out of the bulk structure of thermoelectriccomposite with metal caps according to embodiments of the presentinvention.

The exemplary method further includes removing the top punch on thefirst flat or substantially flat surface of a bottom metal layer made bythe loaded metal powder followed by adding thermoelectric powdermaterial overlying the first flat or substantially flat surface of thebottom metal layer. The exemplary pre-pressing process mentioned aboveis repeated on the added thermoelectric powder material to create asecond flat or substantially flat surface of a thermoelectric thicklayer followed again by removing the corresponding top punch.Furthermore, the exemplary method includes adding metal powdersoverlying the second flat or substantially flat surface and adding toppunch with load to form a top metal layer. Particularly, a tri-layerstructure is formed including the top metal layer over thethermoelectric thick layer over the bottom metal layer. Moreover, theexemplary method includes conducting a co-sintering process applied tothe tri-layer structure to form a bulk-size thermoelectric compositestructure sandwiched by two metal cap layers. In a specific,non-limiting embodiment, the metal powders are Ni powders with fineparticle size of about 5 μm or smaller and the thermoelectric powdermaterial is selected from Mg₂Si powders with ultra fine particle size ofabout 100 nm. In another specific, non-limiting embodiment, the metalpowders are Cr powders with particle size of about 10 μm and thecorresponding thermoelectric powder material is MnSi_(1.73) powder withparticle size of at least smaller than 44 μm.

In a specific, non-limiting embodiment, the co-sintering processincludes ramping temperature from room temperature to about 850° C. witha rate of about 200° C./min. The temperature rise illustratively isachieved with a high pulsed DC current that passes through the sampleand the tooling holding the sample. Illustratively, the pressure appliedonto the tri-layer structure during the sintering can be about 50 MPa.Illustratively, the co-sintering process can be held at 850° C. for 8minutes in an argon environment. In an alternative specific embodiment,the co-sintering process includes applying pressure of about 45 MPa ontothe tri-layer structure and ramping then holding at 300° C. for 5minutes, then ramping up to 850° C. and holding for 60 minutes in anargon environment. In yet another specific embodiment, the co-sinteringprocess includes applying 30 MPa pressure onto the tri-layer structurewhile ramping to and holding at 500° C. for 6 minutes, then ramping upagain to 850° C. and holding for 180 minutes in an argon environment. Instill another specific embodiment, the co-sintering process includesapplying about 30 MPa on the tri-layer structure and ramping to andholding at 500° C. for 6 minutes, then ramping up again to 950° C. andholding for 60 min in an argon environment. In a specific embodiment, athermoelectric material Mg₂Si pellet capped by thick Ni layers is usedto form a plurality of n-type thermoelectric legs with conductor caps.

In another specific, non-limiting embodiment, the method includes analternative co-sintering process for making a pellet of p-typethermoelectric material MnSi_(x) (x≈1.73) capped by two Cr layers. Theco-sintering process can be carried out on a loaded tri-layer structure(e.g., a MnSi_(x) powder layer sandwiched by two Cr powder layers havingflattened interfaces) in the SPS tooling in these steps: 1) withoutramping temperature, ramping pressure to 80 MPa first and then settlingat 15 MPa; 2) holding the pressure at 15 MPa, ramping temperature to300° C. and holding there for 5 minutes; 3) ramping pressure again to 80MPa and ramping temperature to 900° C. and holding there for 15 minutes;4) cooling at a rate of 200° C./min back to room temperature.

In one of specific embodiment, graphite foil is used to prevent thepowder material from sticking to the punch faces during the sinteringprocess. Graphite tooling is also used due to its ability to withstandthe desired sintering temperatures without deforming.

Other exemplary temperatures, pressures, sequences, steps, materials,particle sizes, and device dimensions are described elsewhere herein orsuitably may be envisioned.

According to yet another embodiment, a method of forming athermoelectric device includes preparing a thermoelectric materialincluding a p-type or n-type material and first and second capsrespectively including first and second cap materials respectivelydisposed on either side of the p-type or n-type material, the first andsecond cap materials each respectively including an independentlyselected transition metal. Forming the thermoelectric material caninclude loading a powder precursor of the first cap material into asintering die; assembling one or more punches to the powder precursor ofthe first cap material in the sintering die; and applying a firstpre-load via the one or more punches to the powder precursor of thefirst cap material to form a first pre-pressed structure including afirst substantially flat surface. Forming the thermoelectric materialfurther can include removing a first punch of the one or more punches toexpose the first substantially flat surface; loading a powder precursorof the p-type or n-type material into the sintering die and onto theexposed first substantially flat surface; assembling the first punch tothe powder precursor of the p-type or n-type material in the sinteringdie; and applying a second pre-load via the one or more punches to thefirst pre-pressed structure and the powder precursor of the p-type orn-type material to form a second pre-pressed structure including asecond substantially flat surface. Forming the thermoelectric materialfurther can include removing the first punch to expose the secondsubstantially flat surface; loading a powder precursor of the second capmaterial into the sintering die and onto the exposed secondsubstantially flat surface; assembling the first punch to the powderprecursor of the second cap material in the sintering die; and applyinga third pre-load via the one or more punches to the second pre-pressedstructure and the powder precursor of the second cap to form a thirdpre-pressed structure. Forming the thermoelectric material further caninclude sintering the third pre-pressed structure to form thethermoelectric material; and coupling at least one of the first andsecond caps of the thermoelectric material to an electrical connector.For example, the method is implemented according to at least FIG. 8.

In another example, the method includes selecting the first and secondcap materials so as to respectively include a coefficient of thermalexpansion (CTE) that differs by 20% or less from a CTE of the p-type orn-type material. In another example, the method includes selecting thefirst and second cap materials so as to respectively include acoefficient of thermal expansion (CTE) that differs by 10% or less froma CTE of the p-type or n-type material. In another example, the firstand second cap materials independently include one or more materialsselected from the group consisting of Kovar, Cr, molybdenum, Ni—Fealloy, and Cu—Mo alloy. In another example, the first and second capmaterials independently include one or more materials selected from thegroup consisting of Kovar, Cr, molybdenum, and 50/50 Ni—Fe alloy. Inanother example, the CTE of the p-type or n-type material isapproximately 6-8 ppm/° C. In another example, the first and second capmaterials respectively independently include one or more materialsselected from the group consisting of Ni, Monel, Dura Nickel, a Cu—Nialloy, a Cu—Mo alloy, and Fe. In another example, the first and secondcap materials respectively independently include one or more materialsselected from the group consisting of Ni, Monel, Dura Nickel, Cu—Ni 30,and Cu—Ni 10. In another example, the CTE of the p-type or n-typematerial is approximately 13-17 ppm/° C.

In another example, neither of the first and second cap materialsincludes a silicide.

In another example, the method further includes dicing thethermoelectric material to form a plurality of individual thermoelectriclegs. In another example, the method further includes respectivelycoupling at least one of first and second caps of each of four of theindividual thermoelectric legs to the electrical connector. In anotherexample, the method further includes testing an electrical resistance ofeach of the individual thermoelectric legs, and sorting the individualthermoelectric legs based on the tested electrical resistance.

In another example, the p-type or n-type material includes magnesiumsilicide or manganese silicide. In another example, the p-type or n-typematerial includes tetrahedrite or Mg₂SiSn. In another example, thepowder precursor of the p-type or n-type material includes Mg₂Siultrafine powder formed based on Si and Mg elemental materials, and thepowder precursors of the first and second cap materials include ametallic powder. In another example, the Mg₂Si ultrafine powder ischaracterized by an average particle size of about 10 nm to about 1 μm.In another example, the Mg₂Si ultrafine powder is characterized by anaverage particle size of about 100 nm. In another example, the Mg₂Siultrafine powder is formed and handled in an argon environment withoxygen concentration under 200 ppm before the sintering. In anotherexample, the Mg₂Si ultrafine powder is formed based on the Si and Mgelemental materials using a mechanical alloying ball mill process. Inanother example, the metallic powder includes nickel powder. In anotherexample, the nickel powder is characterized by an average particle sizeof about 100 nm to about 10 μm. In another example, the nickel powder ischaracterized by an average particle size of about 5 μm or less. Inanother example, a particle size ratio of the Mg₂Si ultrafine powder tothe metallic powder is in the range of approximately 1:50 toapproximately 50:1. In another example, a particle size ratio of theMg₂Si ultrafine powder to the metallic powder is approximately 1:20.

In yet another example, the powder precursor of the p-type or n-typematerial includes MnSi_(x) ultrafine powder formed based on Si and Mnelemental materials, and the powder precursors of the first and secondcap materials include a metallic powder. In another example, theMnSi_(x) ultrafine powder is characterized by an average particle sizeof about 44 μm or smaller. In another example, the MnSi_(x) ultrafinepowder is formed and handled in an argon environment before thesintering. In another example, the MnSi_(x) ultrafine powder is formedbased on the Si and Mn elemental materials using a ball mill process. Inanother example, the metallic powder includes chromium powder. Inanother example, the chromium powder is characterized by an averageparticle size ranging from 100 nm to 150 μm in diameter. In anotherexample, the chromium powder is characterized by an average particlesize ranging from 10 μm to 150 μm in diameter. In another example, aparticle size ratio of the MnSi_(x) ultrafine powder to the metallicpowder is in the range of approximately 50:1 to approximately 1:50. Inanother example, a particle size ratio of the MnSi_(x) ultrafine powderto the metallic powder is in the range of approximately 4.4:1 toapproximately 1:3.4. In another example, x is about 1.73.

In yet another example, a particle size ratio of the powder precursor ofthe p-type or n-type material to the powder precursors of the first andsecond cap materials is in the range of approximately 50:1 toapproximately 1:50. In another example, a particle size ratio of thepowder precursor of the p-type or n-type material to the powderprecursors of the first and second cap materials is in the range ofapproximately 4.4:1 to approximately 1:3.4. In another example, aparticle size ratio of the powder precursor of the p-type or n-typematerial to the powder precursors of the first and second cap materialsis in the range of approximately 1:20.

In another example, a thickness of the thermoelectric material isapproximately 0.5 mm to approximately 20 mm. In another example, athickness of the thermoelectric material is approximately 2 mm toapproximately 20 mm. In another example, a thickness of each of thefirst and second caps is approximately 0.2 mm to approximately 2 mm. Inanother example, a thickness of each of the first and second caps isapproximately 1 mm to approximately 2 mm. In another example, athickness of each of the first and second caps is greater thanapproximately 2 mm. In another example, the sintering die includes agraphite material being characterized by a size of 20 mm or greater.

In another example, the first, second, and third pre-loads each are inthe range of approximately 2 MPa to approximately 80 MPa. In anotherexample, the first, second, and third pre-loads each are about 15 MPa.

In another example, sintering the third pre-pressed structure includesapplying a pressure to the third pre-pressed structure while ramping toan intermediate temperature and holding at that temperature for a firstpre-determined period of time; and subsequently ramping to a highertemperature than the intermediate temperature and maintaining thattemperature for a second pre-determined period of time. In anotherexample, sintering the third pre-pressed structure includes ramping to afirst intermediate pressure applied to the third pre-pressed structure;subsequently reducing to a second intermediate pressure; subsequentlyramping to an intermediate temperature and holding at that temperaturefor a first pre-determined period of time; and subsequently ramping to ahigher temperature than the intermediate temperature and holding at thattemperature for a second pre-determined period of time. In anotherexample, the sintering further includes, subsequently to ramping to theintermediate temperature, ramping to a higher pressure than the secondintermediate pressure.

In yet another example, the sintering includes applying a pressure viathe one or more punches to the third pre-pressed structure while:ramping a temperature of the third pre-pressed structure from roomtemperature to about 850° C. at a rate of about 200° C./min; andsubsequently holding the third pre-pressed structure at about 850° C.for a pre-determined period of time. In another example, thepre-determined period of time is about 8 minutes. In another example,the pressure is about 50 MPa. In another example, the ramping isachieved with a pulsed DC current that passes through the thirdpre-pressed structure, the sintering die, and the one or more punches.

In yet another example, the sintering includes applying a pressure viathe one or more punches to the third pre-pressed structure while:ramping a temperature of the third pre-pressed structure from roomtemperature to about 300° C.; subsequently holding the third pre-pressedstructure at about 300° C. for a first pre-determined period of time;subsequently ramping the temperature of the third pre-pressed structurefrom 300° C. to about 850° C.; and subsequently holding the thirdpre-pressed structure at about 850° C. for a second pre-determinedperiod of time. In another example, the first pre-determined period oftime is about 5 minutes, and the second pre-determined period of time isabout 60 minutes. In another example, the pressure is about 45 MPa.

In yet another example, the sintering includes applying a pressure viathe one or more punches to the third pre-pressed structure while:ramping a temperature of the third pre-pressed structure from roomtemperature to about 500° C.; subsequently holding the third pre-pressedstructure at about 500° C. for a first pre-determined period of time;subsequently ramping the temperature of the third pre-pressed structurefrom 300° C. to about 850° C.; and subsequently holding the thirdpre-pressed structure at about 850° C. for a second pre-determinedperiod of time. In another example, the first pre-determined period oftime is about 6 minutes, and the second pre-determined period of time isabout 180 minutes. In another example, the pressure is about 30 MPa.

In yet another example, the sintering includes applying a pressure viathe one or more punches to the third pre-pressed structure while:ramping a temperature of the third pre-pressed structure from roomtemperature to about 500° C.; subsequently holding the third pre-pressedstructure at about 500° C. for a first pre-determined period of time;subsequently ramping the temperature of the third pre-pressed structurefrom 500° C. to about 950° C.; and subsequently holding the thirdpre-pressed structure at about 950° C. for a second pre-determinedperiod of time. In another example, the first pre-determined period oftime is about 6 minutes, and the second pre-determined period of time isabout 60 minutes. In another example, the pressure is about 30 MPa.

In yet another example, the sintering includes: via the one or morepunches, ramping a pressure to the third pre-pressed structure to about80 MPa and then settling the pressure at 15 MPa; at the pressure of 15MPa, subsequently ramping a temperature of the third pre-pressedstructure from room temperature to about 300° C. and then holding thethird pre-pressed structure at about 300° C. for a first pre-determinedperiod of time; at the temperature of 300° C., subsequently ramping thepressure to about 80 MPa; at the pressure of 80 MPa, subsequentlyramping the temperature of the third pre-pressed structure from 300° C.to about 900° C.; and subsequently holding the third pre-pressedstructure at about 900° C. for a second pre-determined period of time.In another example, the first pre-determined period of time is about 5minutes, and the second pre-determined period of time is about 15minutes.

In another example, the sintering further includes, after holding thethird pre-pressed structure at about 900° C. for the secondpre-determined period of time, cooling the third pre-pressed structureat a rate of about 200° C./minute.

In yet another example, a cross-sectional area of the thermoelectricmaterial is in the range of approximately 1.8 mm×1.8 mm andapproximately 3.6 mm×1.8 mm; a thickness of the p-type or n-typematerial is in the range of approximately 0.5 mm to approximately 2.5mm; and an electrical resistance of the thermoelectric material is inthe range of approximately 2 mOhm to approximately 10 mOhm.

In another example, a thermoelectric device is provided that is preparedusing the method of any one or more of the foregoing exemplary methods.For example, the device is implemented according to at least FIG. 3,FIGS. 4A-4D, FIGS. 5A-5B, FIG. 6, FIG. 7, FIG. 8, or FIGS. 9A-9B. Inanother example, such a thermoelectric device is used to generate acurrent or voltage. In another example, such a thermoelectric device isused to heat or cool a body to which the thermoelectric device iscoupled.

According to yet another embodiment, a thermoelectric device includes athermoelectric material including a p-type or n-type material and firstand second caps respectively including first and second cap materialsrespectively disposed on either side of the p-type or n-type material,the first and second cap materials each respectively including anindependently selected transition metal. The thermoelectric material canbe formed by co-sintering a powder precursor of the first cap material,a powder precursor of the p-type or n-type material, and a powderprecursor of the second cap material in a sintering die. A particle sizeratio of the powder precursor of the p-type or n-type material to thepowder precursors of the first and second cap materials can be in therange of approximately 1:1 to approximately 1:50. The device also caninclude an electrical connector, at least one of the first and secondcaps of the thermoelectric material being coupled to the electricalconnector. For example, the device is implemented according to at leastFIG. 3, FIGS. 4A-4D, FIGS. 5A-5B, FIG. 6, FIG. 7, FIG. 8, or FIGS.9A-9B.

In another example, the first and second cap materials respectivelyinclude a coefficient of thermal expansion (CTE) that differs by 20% orless from a CTE of the p-type or n-type material. In another example,the first and second cap materials respectively include a coefficient ofthermal expansion (CTE) that differs by 10% or less from a CTE of thep-type or n-type material. In another example, the first and second capmaterials independently include one or more materials selected from thegroup consisting of Kovar, Cr, molybdenum, Ni—Fe alloy, and Cu—Mo alloy.In another example, the first and second cap materials independentlyinclude one or more materials selected from the group consisting ofKovar, Cr, molybdenum, and 50/50 Ni—Fe alloy. In another example, theCTE of the p-type or n-type material is approximately 6-8 ppm/° C. Inanother example, the first and second cap materials respectivelyindependently include one or more materials selected from the groupconsisting of Ni, Monel, Dura Nickel, a Cu—Ni alloy, a Cu—Mo alloy, andFe. In another example, the first and second cap materials respectivelyindependently include one or more materials selected from the groupconsisting of Ni, Monel, Dura Nickel, Cu—Ni 30, and Cu—Ni 10. In anotherexample, the CTE of the p-type or n-type material is approximately 13-17ppm/° C. In another example, neither of the first and second capmaterials includes a silicide. In another example, the thermoelectricdevice further includes a plurality of individual thermoelectric legsrespectively formed by dicing the thermoelectric material. In anotherexample, at least one of first and second caps of each of four of theindividual thermoelectric legs is coupled to the electrical connector.In another example, a cross-sectional area of each of the individualthermoelectric legs is in the range of approximately 1.8 mm×1.8 mm andapproximately 3.6 mm×1.8 mm; a thickness of the p-type or n-typematerial is in the range of approximately 0.5 mm to approximately 2.5mm; and an electrical resistance of each of the individualthermoelectric legs is in the range of approximately 2 mOhm toapproximately 10 mOhm.

In another example, the p-type or n-type material includes magnesiumsilicide or manganese silicide. In another example, the p-type or n-typematerial includes tetrahedrite or Mg₂SiSn. In another example, thepowder precursor of the p-type or n-type material includes Mg₂Siultrafine powder formed based on Si and Mg elemental materials, andwherein the powder precursors of the first and second cap materialsinclude a metallic powder. In another example, the Mg₂Si ultrafinepowder is characterized by an average particle size of about 10 nm toabout 1 μm. In another example, the Mg₂Si ultrafine powder ischaracterized by an average particle size of about 100 nm. In anotherexample, the Mg₂Si ultrafine powder is formed and handled in an argonenvironment with oxygen concentration under 200 ppm before thesintering. In another example, the Mg₂Si ultrafine powder is formedbased on the Si and Mg elemental materials using a mechanical alloyingball mill process. In another example, the metallic powder includesnickel powder. In another example, the nickel powder is characterized byan average particle size of about 100 nm to about 10 μm. In anotherexample, the nickel powder is characterized by an average particle sizeof about 5 μm or less. In another example, a particle size ratio of theMg₂Si ultrafine powder to the metallic powder is approximately 1:20.

In yet another example, the powder precursor of the p-type or n-typematerial includes MnSi_(x) ultrafine powder formed based on Si and Mnelemental materials, and the powder precursors of the first and secondcap materials include a metallic powder. In another example, theMnSi_(x) ultrafine powder is characterized by an average particle sizeof about 44 μm or smaller. In another example, the MnSi_(x) ultrafinepowder is formed and handled in an argon environment before thesintering. In another example, the MnSi_(x) ultrafine powder is formedbased on the Si and Mn elemental materials using a ball mill process. Inanother example, the metallic powder includes chromium powder. Inanother example, the chromium powder is characterized by an averageparticle size ranging from 100 nm to 150 μm in diameter. In anotherexample, the chromium powder is characterized by an average particlesize ranging from 10 μm to 150 μm in diameter. In another example, aparticle size ratio of the MnSi_(x) ultrafine powder to the metallicpowder is in the range of approximately 4.4:1 to approximately 1:3.4. Inanother example, x is approximately 1.73.

In yet another example, a particle size ratio of the powder precursor ofthe p-type or n-type material to the powder precursors of the first andsecond cap materials is in the range of approximately 4.4:1 toapproximately 1:3.4. In another example, a particle size ratio of thepowder precursor of the p-type or n-type material to the powderprecursors of the first and second cap materials is in the range ofapproximately 1:20. In another example, a thickness of thethermoelectric material is approximately 0.5 mm to approximately 20 mm.In another example, a thickness of the thermoelectric material isapproximately 2 mm to approximately 20 mm. In another example, athickness of each of the first and second caps is approximately 0.2 mmto approximately 2 mm. In another example, a thickness of each of thefirst and second caps is approximately 1 mm to approximately 2 mm. Inanother example, a thickness of each of the first and second caps isgreater than approximately 2 mm.

In yet another example, the co-sintering includes: loading a powderprecursor of the first cap material into a sintering die; assembling oneor more punches to the powder precursor of the first cap material in thesintering die; and applying a first pre-load via the one or more punchesto the powder precursor of the first cap material to form a firstpre-pressed structure including a first substantially flat surface. Theco-sintering further can include removing a first punch of the one ormore punches to expose the first substantially flat surface; loading apowder precursor of the p-type or n-type material into the sintering dieand onto the exposed first substantially flat surface; assembling thefirst punch to the powder precursor of the p-type or n-type material inthe sintering die; and applying a second pre-load via the one or morepunches to the first pre-pressed structure and the powder precursor ofthe p-type or n-type material to form a second pre-pressed structureincluding a second substantially flat surface. The co-sintering furthercan include removing the first punch to expose the second substantiallyflat surface; loading a powder precursor of the second cap material intothe sintering die and onto the exposed second substantially flatsurface; assembling the first punch to the powder precursor of thesecond cap material in the sintering die; and applying a third pre-loadvia the one or more punches to the second pre-pressed structure and thepowder precursor of the second cap to form a third pre-pressedstructure. The co-sintering further can include sintering the thirdpre-pressed structure to form the thermoelectric material.

In another example, any of the aforementioned thermoelectric devices canbe used to generate a current or voltage. In another example, any of theaforementioned thermoelectric devices can be used to heat or cool a bodyto which the thermoelectric device is coupled.

According to still another embodiment, a method of forming athermoelectric device can include providing a thermoelectric materialincluding a p-type or n-type material and first and second capsrespectively including first and second cap materials respectivelydisposed on either side of the p-type or n-type material, the first andsecond cap materials each respectively including an independentlyselected transition metal, wherein a thickness of each of the first andsecond caps is approximately 0.2 mm to approximately 2 mm. The methodfurther can include dicing the thermoelectric material to form aplurality of individual thermoelectric legs; testing an electricalresistance of each of the individual thermoelectric legs; sorting theindividual thermoelectric legs based on the tested electricalresistance; and coupling at least one of the first and second caps of atleast one of the sorted individual thermoelectric legs to an electricalconnector. For example, the method is implemented according to at leastFIG. 6.

In another example, the method includes selecting the first and secondcap materials so as to respectively include a coefficient of thermalexpansion (CTE) that differs by 20% or less from a CTE of the p-type orn-type material. In another example, the method includes selecting thefirst and second cap materials so as to respectively include acoefficient of thermal expansion (CTE) that differs by 10% or less froma CTE of the p-type or n-type material. In another example, the firstand second cap materials independently include one or more materialsselected from the group consisting of Kovar, Cr, molybdenum, Ni—Fealloy, and Cu—Mo alloy. In another example, the first and second capmaterials independently include one or more materials selected from thegroup consisting of Kovar, Cr, molybdenum, and 50/50 Ni—Fe alloy. Inanother example, the CTE of the p-type or n-type material isapproximately 6-8 ppm/° C. In another example, the first and second capmaterials respectively independently include one or more materialsselected from the group consisting of Ni, Monel, Dura Nickel, a Cu—Nialloy, a Cu—Mo alloy, and Fe. In another example, the first and secondcap materials respectively independently include one or more materialsselected from the group consisting of Ni, Monel, Dura Nickel, Cu—Ni 30,and Cu—Ni 10. In another example, the CTE of the p-type or n-typematerial is approximately 13-17 ppm/° C. In another example, neither ofthe first and second cap materials includes a silicide. In anotherexample, the p-type or n-type material includes magnesium silicide ormanganese silicide. In another example, the p-type or n-type materialincludes tetrahedrite or Mg₂SiSn.

In another example, forming the thermoelectric material includes:loading a powder precursor of the first cap material into a sinteringdie; assembling one or more punches to the powder precursor of the firstcap material in the sintering die; and applying a first pre-load via theone or more punches to the powder precursor of the first cap material toform a first pre-pressed structure including a first substantially flatsurface. Forming the thermoelectric material also can include removing afirst punch of the one or more punches to expose the first substantiallyflat surface; loading a powder precursor of the p-type or n-typematerial into the sintering die and onto the exposed first substantiallyflat surface; assembling the first punch to the powder precursor of thep-type or n-type material in the sintering die; and applying a secondpre-load via the one or more punches to the first pre-pressed structureand the powder precursor of the p-type or n-type material to form asecond pre-pressed structure including a second substantially flatsurface. Forming the thermoelectric material also can include removingthe first punch to expose the second substantially flat surface; loadinga powder precursor of the second cap material into the sintering die andonto the exposed second substantially flat surface; assembling the firstpunch to the powder precursor of the second cap material in thesintering die; and applying a third pre-load via the one or more punchesto the second pre-pressed structure and the powder precursor of thesecond cap to form a third pre-pressed structure. Forming thethermoelectric material further can include sintering the thirdpre-pressed structure to form the thermoelectric material.

In another example, the powder precursor of the p-type or n-typematerial includes Mg₂Si ultrafine powder formed based on Si and Mgelemental materials, and the powder precursors of the first and secondcap materials include a metallic powder. In another example, the Mg₂Siultrafine powder is characterized by an average particle size of about10 nm to about 1 μm. In another example, the Mg₂Si ultrafine powder ischaracterized by an average particle size of about 100 nm. In anotherexample, the Mg₂Si ultrafine powder is formed and handled in an argonenvironment with oxygen concentration under 200 ppm before thesintering. In another example, the Mg₂Si ultrafine powder is formedbased on the Si and Mg elemental materials using a mechanical alloyingball mill process. In another example, the metallic powder includesnickel powder. In another example, the nickel powder is characterized byan average particle size of about 100 nm to about 10 μm. In anotherexample, the nickel powder is characterized by an average particle sizeof about 5 μm or less. In another example, a particle size ratio of theMg₂Si ultrafine powder to the metallic powder is in the range ofapproximately 1:50 to approximately 50:1. In another example, a particlesize ratio of the Mg₂Si ultrafine powder to the metallic powder isapproximately 1:20.

In yet another example, the powder precursor of the p-type or n-typematerial includes MnSi_(x) ultrafine powder formed based on Si and Mnelemental materials, and the powder precursors of the first and secondcap materials include a metallic powder. In another example, theMnSi_(x) ultrafine powder is characterized by an average particle sizeof about 44 μm or smaller. In another example, the MnSi_(x) ultrafinepowder is formed and handled in an argon environment before thesintering. In another example, the MnSi_(x) ultrafine powder is formedbased on the Si and Mn elemental materials using a ball mill process. Inanother example, the metallic powder includes chromium powder. Inanother example, the chromium powder is characterized by an averageparticle size ranging from 100 nm to 150 μm in diameter. In anotherexample, chromium powder is characterized by an average particle sizeranging from 10 μm to 150 μm in diameter. In another example, a particlesize ratio of the MnSi_(x) ultrafine powder to the metallic powder is inthe range of approximately 50:1 to approximately 1:50. In anotherexample, a particle size ratio of the MnSi_(x) ultrafine powder to themetallic powder is in the range of approximately 4.4:1 to approximately1:3.4. In another example, x is about 1.73.

In yet another example, a particle size ratio of the powder precursor ofthe p-type or n-type material to the powder precursors of the first andsecond cap materials is in the range of approximately 50:1 toapproximately 1:50. In another example, a particle size ratio of thepowder precursor of the p-type or n-type material to the powderprecursors of the first and second cap materials is in the range ofapproximately 4.4:1 to approximately 1:3.4. In another example, aparticle size ratio of the powder precursor of the p-type or n-typematerial to the powder precursors of the first and second cap materialsis in the range of approximately 1:20. In another example, a thicknessof the thermoelectric material is approximately 0.5 mm to approximately20 mm. In another example, a thickness of the thermoelectric material isapproximately 2 mm to approximately 20 mm. In another example, athickness of each of the first and second caps is approximately 1 mm toapproximately 2 mm. In another example, a thickness of each of the firstand second caps is greater than approximately 2 mm. In another example,the sintering die includes a graphite material characterized by a sizeof 20 mm or greater. In another example, the first, second, and thirdpre-loads each are in the range of approximately 2 MPa to approximately80 MPa. In another example, the first, second, and third pre-loads eachare about 15 MPa.

In another example, sintering the third pre-pressed structure includes:applying a pressure to the third pre-pressed structure while ramping toan intermediate temperature and holding at that temperature for a firstpre-determined period of time; and subsequently ramping to a highertemperature than the intermediate temperature and maintaining thattemperature for a second pre-determined period of time. In anotherexample, sintering the third pre-pressed structure includes: ramping toa first intermediate pressure applied to the third pre-pressedstructure; subsequently reducing to a second intermediate pressure;subsequently ramping to an intermediate temperature and holding at thattemperature for a first pre-determined period of time; and subsequentlyramping to a higher temperature than the intermediate temperature andholding at that temperature for a second pre-determined period of time.In another example, the sintering includes subsequently to ramping tothe intermediate temperature, ramping to a higher pressure than thesecond intermediate pressure.

In another example, the sintering includes applying a pressure via theone or more punches to the third pre-pressed structure while: ramping atemperature of the third pre-pressed structure from room temperature toabout 850° C. at a rate of about 200° C./min; and subsequently holdingthe third pre-pressed structure at about 850° C. for a pre-determinedperiod of time. In another example, the pre-determined period of time isabout 8 minutes. In another example, the pressure is about 50 MPa. Inanother example, the ramping is achieved with a pulsed DC current thatpasses through the third pre-pressed structure, the sintering die, andthe one or more punches.

In another example, the sintering includes applying a pressure via theone or more punches to the third pre-pressed structure while: ramping atemperature of the third pre-pressed structure from room temperature toabout 300° C.; subsequently holding the third pre-pressed structure atabout 300° C. for a first pre-determined period of time; subsequentlyramping the temperature of the third pre-pressed structure from 300° C.to about 850° C.; and subsequently holding the third pre-pressedstructure at about 850° C. for a second pre-determined period of time.In another example, the first pre-determined period of time is about 5minutes, and the second pre-determined period of time is about 60minutes. In another example, the pressure is about 45 MPa.

In yet another example, the sintering includes applying a pressure viathe one or more punches to the third pre-pressed structure while:ramping a temperature of the third pre-pressed structure from roomtemperature to about 500° C.; subsequently holding the third pre-pressedstructure at about 500° C. for a first pre-determined period of time;subsequently ramping the temperature of the third pre-pressed structurefrom 300° C. to about 850° C.; and subsequently holding the thirdpre-pressed structure at about 850° C. for a second pre-determinedperiod of time. In another example, the first pre-determined period oftime is about 6 minutes, and the second pre-determined period of time isabout 180 minutes. In another example, the pressure is about 30 MPa.

In still another example, the sintering includes applying a pressure viathe one or more punches to the third pre-pressed structure while:ramping a temperature of the third pre-pressed structure from roomtemperature to about 500° C.; subsequently holding the third pre-pressedstructure at about 500° C. for a first pre-determined period of time;subsequently ramping the temperature of the third pre-pressed structurefrom 500° C. to about 950° C.; and subsequently holding the thirdpre-pressed structure at about 950° C. for a second pre-determinedperiod of time. In another example, the first pre-determined period oftime is about 6 minutes, and wherein the second pre-determined period oftime is about 60 minutes. In another example, the pressure is about 30MPa.

In yet another example, the sintering includes: via the one or morepunches, ramping a pressure to the third pre-pressed structure to about80 MPa and then settling the pressure at 15 MPa; at the pressure of 15MPa, subsequently ramping a temperature of the third pre-pressedstructure from room temperature to about 300° C. and then holding thethird pre-pressed structure at about 300° C. for a first pre-determinedperiod of time; at the temperature of 300° C., subsequently ramping thepressure to about 80 MPa; at the pressure of 80 MPa, subsequentlyramping the temperature of the third pre-pressed structure from 300° C.to about 900° C.; and subsequently holding the third pre-pressedstructure at about 900° C. for a second pre-determined period of time.In another example, the first pre-determined period of time is about 5minutes, and wherein the second pre-determined period of time is about15 minutes. In another example, the sintering includes after holding thethird pre-pressed structure at about 900° C. for the secondpre-determined period of time, cooling the third pre-pressed structureat a rate of about 200° C./min.

In another example, a cross-sectional area of the sorted individualthermoelectric legs is in the range of approximately 1.8 mm×1.8 mm andapproximately 3.6 mm×1.8 mm; a thickness of the p-type or n-typematerial is in the range of approximately 0.5 mm to approximately 2.5mm; and an electrical resistance of the at least one of the sortedthermoelectric legs coupled to the electrical connector is in the rangeof approximately 2 mOhm to approximately 10 mOhm.

In another example, a thermoelectric device is provided that is preparedusing the method of any one or more of the foregoing exemplary methods.For example, the device is implemented according to at least FIG. 3,FIGS. 4A-4D, FIGS. 5A-5B, FIG. 6, FIG. 7, FIG. 8, or FIGS. 9A-9B. Inanother example, such a thermoelectric device is used to generate acurrent or voltage. In another example, such a thermoelectric device isused to heat or cool a body to which the thermoelectric device iscoupled.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.For example, various embodiments and/or examples of the presentinvention can be combined. Accordingly, it is to be understood that theinvention is not to be limited by the specific illustrated embodiments,but only by the scope of the appended claims.

What is claimed is:
 1. A thermoelectric device including: athermoelectric material including a p-type or n-type material and firstand second caps respectively including first and second cap materialsrespectively disposed on either side of the p-type or n-type material,the first and second cap materials each respectively including anindependently selected transition metal, the thermoelectric materialbeing formed by co-sintering a powder precursor of the first capmaterial, a powder precursor of the p-type or n-type material, and apowder precursor of the second cap material in a sintering die, whereina particle size ratio of the powder precursor of the p-type or n-typematerial to the powder precursors of the first and second cap materialsis in the range of approximately 1:1 to approximately 1:50; and anelectrical connector, at least one of the first and second caps of thethermoelectric material being coupled to the electrical connector. 2.The device of claim 1, wherein the first and second cap materialsrespectively include a coefficient of thermal expansion (CTE) thatdiffers by 20% or less from a CTE of the p-type or n-type material. 3.The device of claim 1, wherein the first and second cap materialsrespectively include a coefficient of thermal expansion (CTE) thatdiffers by 10% or less from a CTE of the p-type or n-type material. 4.The device of claim 3, wherein the first and second cap materialsindependently include one or more materials selected from the groupconsisting of Kovar, Cr, molybdenum, Ni—Fe alloy, and Cu—Mo alloy. 5.The device of claim 3, wherein the first and second cap materialsindependently include one or more materials selected from the groupconsisting of Kovar, Cr, molybdenum, and 50/50 Ni—Fe alloy.
 6. Thedevice of claim 5, wherein the CTE of the p-type or n-type material isapproximately 6-8 ppm/° C.
 7. The device of claim 3, wherein the firstand second cap materials respectively independently include one or morematerials selected from the group consisting of Ni, Monel, Dura Nickel,a Cu—Ni alloy, a Cu—Mo alloy, and Fe.
 8. The device of claim 3, whereinthe first and second cap materials respectively independently includeone or more materials selected from the group consisting of Ni, Monel,Dura Nickel, Cu—Ni 30, and Cu—Ni
 10. 9. The device of claim 8, whereinthe CTE of the p-type or n-type material is approximately 13-17 ppm/° C.10. The device of claim 1, wherein neither of the first and second capmaterials includes a silicide.
 11. The device of claim 1, including aplurality of individual thermoelectric legs respectively formed bydicing the thermoelectric material.
 12. The device of claim 11, whereinat least one of first and second caps of each of four of the individualthermoelectric legs is coupled to the electrical connector.
 13. Thedevice of claim 11, wherein: a cross-sectional area of each of theindividual thermoelectric legs is in the range of approximately 1.8mm×1.8 mm and approximately 3.6 mm×1.8 mm; a thickness of the p-type orn-type material is in the range of approximately 0.5 mm to approximately2.5 mm; and an electrical resistance of each of the individualthermoelectric legs is in the range of approximately 2 mOhm toapproximately 10 mOhm.
 14. The device of claim 1, wherein the p-type orn-type material includes magnesium silicide or manganese silicide. 15.The device of claim 14, wherein the powder precursor of the p-type orn-type material includes Mg₂Si ultrafine powder formed based on Si andMg elemental materials, and wherein the powder precursors of the firstand second cap materials include a metallic powder.
 16. The device ofclaim 15, the Mg₂Si ultrafine powder being characterized by an averageparticle size of about 10 nm to about 1 μm.
 17. The device of claim 15,the Mg₂Si ultrafine powder being characterized by an average particlesize of about 100 nm.
 18. The device of claim 17, wherein the Mg₂Siultrafine powder is formed and handled in an argon environment withoxygen concentration under 200 ppm before the sintering.
 19. The deviceof claim 15, wherein the Mg₂Si ultrafine powder is formed based on theSi and Mg elemental materials using a mechanical alloying ball millprocess.
 20. The device of claim 15, wherein the metallic powderincludes nickel powder.
 21. The device of claim 20, the nickel powderbeing characterized by an average particle size of about 100 nm to about10 μm.
 22. The device of claim 20, the nickel powder being characterizedby an average particle size of about 5 μm or less.
 23. The device ofclaim 15, wherein a particle size ratio of the Mg₂Si ultrafine powder tothe metallic powder is approximately 1:20.
 24. The device of claim 14,wherein the powder precursor of the p-type or n-type material includesMnSi_(x) ultrafine powder formed based on Si and Mn elemental materials,and wherein the powder precursors of the first and second cap materialsinclude a metallic powder.
 25. The device of claim 24, the MnSi_(x)ultrafine powder being characterized by an average particle size ofabout 44 μm or smaller.
 26. The device of claim 24, wherein the MnSi_(x)ultrafine powder is formed and handled in an argon environment beforethe sintering.
 27. The device of claim 24, wherein the MnSi_(x)ultrafine powder is formed based on the Si and Mn elemental materialsusing a ball mill process.
 28. The device of claim 24, wherein themetallic powder includes chromium powder.
 29. The device of claim 28,the chromium powder being characterized by an average particle sizeranging from 100 nm to 150 μm in diameter.
 30. The device of claim 28,the chromium powder being characterized by an average particle sizeranging from 10 μm to 150 μm in diameter.
 31. The device of claim 24,wherein a particle size ratio of the MnSi_(x) ultrafine powder to themetallic powder is in the range of approximately 4.4:1 to approximately1:3.4.
 32. The device of claim 24, wherein x is approximately 1.73. 33.The device of claim 1, wherein the p-type or n-type material includestetrahedrite or Mg₂SiSn.
 34. The device of claim 1, wherein a particlesize ratio of the powder precursor of the p-type or n-type material tothe powder precursors of the first and second cap materials is in therange of approximately 4.4:1 to approximately 1:3.4.
 35. The device ofclaim 1, wherein a particle size ratio of the powder precursor of thep-type or n-type material to the powder precursors of the first andsecond cap materials is in the range of approximately 1:20.
 36. Thedevice of claim 1, wherein a thickness of the thermoelectric material isapproximately 0.5 mm to approximately 20 mm.
 37. The device of claim 1,wherein a thickness of the thermoelectric material is approximately 2 mmto approximately 20 mm.
 38. The device of claim 1, wherein a thicknessof each of the first and second caps is approximately 0.2 mm toapproximately 2 mm.
 39. The device of claim 1, wherein a thickness ofeach of the first and second caps is approximately 1 mm to approximately2 mm.
 40. The device of claim 1, wherein a thickness of each of thefirst and second caps is greater than approximately 2 mm.
 41. The deviceof claim 1, wherein the co-sintering includes: loading a powderprecursor of the first cap material into a sintering die; assembling oneor more punches to the powder precursor of the first cap material in thesintering die; applying a first pre-load via the one or more punches tothe powder precursor of the first cap material to form a firstpre-pressed structure including a first substantially flat surface;removing a first punch of the one or more punches to expose the firstsubstantially flat surface; loading a powder precursor of the p-type orn-type material into the sintering die and onto the exposed firstsubstantially flat surface; assembling the first punch to the powderprecursor of the p-type or n-type material in the sintering die;applying a second pre-load via the one or more punches to the firstpre-pressed structure and the powder precursor of the p-type or n-typematerial to form a second pre-pressed structure including a secondsubstantially flat surface; removing the first punch to expose thesecond substantially flat surface; loading a powder precursor of thesecond cap material into the sintering die and onto the exposed secondsubstantially flat surface; assembling the first punch to the powderprecursor of the second cap material in the sintering die; applying athird pre-load via the one or more punches to the second pre-pressedstructure and the powder precursor of the second cap to form a thirdpre-pressed structure; and sintering the third pre-pressed structure toform the thermoelectric material.